The term dyke or dike is a slang noun meaning lesbian; it is also a slang adjective describing things associated with lesbianism. It originated as a derogatory label for a masculine, tomboyish, or butch woman; while this usage still exists, the term "dyke" has been reappropriated to an extent as a word implying assertiveness and toughness, or simply as a neutral synonym for lesbian.
The origin of the term is obscure and many theories have been proposed. The Oxford English Dictionary notes the first attestation as Berrey and Van den Bark's 1942 American Thesaurus of Slang. There, "dike" was the more common term. From the mid-19th century to the early 20th century, "dike" had been American slang for a well-dressed man, with "diked out" and "out on a dike" indicating a young man was in his best clothes and ready for a night on the town. The etymology of that term is also obscure, but may have originated as a Virginian variant of "deck" and "decked out".
However, the term "bulldyker" preceded "dyke" in print, appearing in Harlem Renaissance novels in the 1920s. Claude McKay's 1928 Home to Harlem includes the passage that lesbians are "what we calls bulldyker in Harlem... I don't understan'... a bulldyking woman." From the context in the novel, the word was considered crude and pejorative at the time. This may be related to the late-19th-century slang use of "dike" ("ditch") for the vulva. "Bull" ("male cattle") being used in the sense of "masculine" and "aggressive" (e.g., in bullish), a "bulldyke" would have implied (with similar levels of offensiveness) a "masculine cunt". Other theories include that "bulldyker" derived from "morphadike", a dialect variant of "hermaphrodite", used for homosexuals in the early 20th century; that it was a term for stud bulls and originally applied to sexually successful men; or that it was a dialectical corruption of the rebel Celtic queen Boadicea.
In an investigative study by Julia Stanley, she theorizes that the source of these varying definitions stems from gender-determined sub-dialects. Homosexuality in America is a “subculture with its own language.” As such, a special vocabulary is developed by its members. Previously, male homosexuals defined dyke as lesbian without derogation. A bull dyke was also defined as a lesbian without further distinction. For female homosexuals of the community, however, a dyke is an extremely masculine, easily identified lesbian, given to indiscretion. Bull dyke is an extension of this term, with the addition of this person described as nasty, obnoxiously aggressive, and overly demonstrative of her hatred of men.
In an alternative investigation, Susan Krantz discusses the etymology of bulldyke, with derivations of the Middle English “falsehood” for bull and “dick” for dyke (Farmer and Henley 1891). Therefore, a possible origin for a masculine lesbian comes from “bulldicker” that could specifically mean “fake penis,” denoting a “false man.” Further speculation talks of the synonymous term “bulldagger.” Here, dagger also alludes to the male genitalia and bull referring to false rather than man. The earliest account of dagger in this context stems from an account in 1348 by Henry Knighton.
In the 1950s, the word “dyke” was used as a derogatory term used by straight people and lesbians who were most likely to be able to move up in social class. They used this term to identify crude, rough bar lesbians. In the 1970s, a poem called “Edward the Dyke” was published by Women’s Press Collective. This empowered the lesbian community because they had never heard the term “dyke” used officially, due to the fact that it was only used as a derogatory term against them. Because of the exposure of the word to the public, the term “dyke” was officially taken up by the lesbian community in the 1970s. 
Dyke’s definition has positively changed over time. Most members of the community have dropped “bull” from the term to use it as a positive identifier of one that displays toughness or as a simple, generic term for all lesbians. Due to this, the abbreviation does not carry the negative connotations of the full word like previously did. In reference to Paula Blank's account regarding lesbian etymology, she calls for taking ownership of lesbian and similar vocabulary. Taking control of the language allows for the prevention of setting words apart, which inevitably leads to the alienation or isolation.
In the late 20th and early 21st century the term “dyke” was claimed by many lesbian as a term of pride and empowerment. Matters came to a head when the United States Patent and Trademark office denied lesbian motorcycle group “Dykes on Bikes” a trademark for its name, on the ground “dyke” was offensive, derogatory and disparaging to lesbian group. However, the office reversed itself and permitted the group to register its names the after lawyers appealed and submitting hundreds of pages to show the slang word does not disparage lesbians.. “Dykes on Bikes” group declared a victory on December 8, 2005 and slowly gain international recognition for leading the city’s gay parade.
Much of the gay slang that is used today was seen by the public as “self-destructive” and “demeaning” especially when it was used within the LGBTQ+ community. In 1969, people in the gay community began to march in the streets to try and change the connotation of gay slang terms. If used, terms such as “dyke” and “faggot” were used to identify people as political activists for the gay community. During this time, “dyke” referred to a woman committed to revolution, the most radical position. A surge of feminism in the dyke community led to “dyke separatism”, which emphasized that lesbian women should consider themselves to be separate from men and their ideas and movements.
"Dyke Marches" have become a popular Pride event nationwide. They are generally non-commercial, often in sharp contrast to corporate-sponsored pride events, and are usually inclusive of lesbian, bi, and trans women. The Dyke March held in Boston is known for having a spirit of unity and inclusiveness. Its mission is “to provide a dynamic and welcoming space for participants of all sexualities, gender expressions, races, ages, ethnicities, sizes, economic backgrounds, and physical abilities.”
A dyke bar is a term used to describe any bar or club in which lesbians often attend, but can also indicate a "tougher" establishment (in terms of the patrons or environment). As with the stand-alone word "dyke," the term is considered not only slang, but a potential slur when used by non-LGBT persons.
In the articles 'The Only Dykey One' Lucy Jones claims that consideration of lesbian culture is core to an understanding of "lesbian" identity construction. At one point, one can tell very much if a woman is a dyke because of how a woman chose to dress. If a woman diverges from the ideal of being feminine (makeup, jewelry, feminine clothing, attractive body, thin, large breasts, etc.), one would suspect that they are dyke or bisexual. Women who were less stereotypically “feminine” or coded as "masculine" were likely to be a butch lesbian.
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- Etymology of dyke on the Online Etymology Dictionary