A cockblock is a slang term for an intentional, or sometimes unintentional action that serves to prevent someone from having sexual intercourse with a partner. Similarly, cockbend is a slang for and intentional, or sometimes unintentional action that serves to prevent someone from having sexual intercourse with the intended partner but with someone else. 
Such behavior is often motivated by jealousy, or competitiveness, although it is sometimes accidental, or inadvertent. A cockblock or cockblocker is a person who engages in such obstruction or intervention.
Social research has documented norms among male peer groups that view "cockblock" behavior as negative, which may make men less likely to intervene as bystanders in situations of interpersonal violence. The terms appears to date at least to 1972, when Edith Folb documented its use by urban black teenagers.
- Folb, Edith. Runnin’ down some lines, Society, Volume 18, Number 1 (1980), 63–71 ("There are a number of expressions in the vernacular that characterize one person's attempt to interrupt, impede, or totally sabotage another's action – to cock block ...")
- Stark, John et al. The Dictionary of Love, p. 67 (2008)
- Sallee, Margaret W. & Frank Harris III. Gender performance in qualitative studies of masculinities, Qualitative Research 11.4 (2011): 409–429, at 419–20 (including research subject interview defining term)
- Wilser, Jeff. The Maxims of Manhood: 100 Rules Every Real Man Must Live By, pp. 171–72 (2009)
- Casey, Erin A. & Kristin Ohler. "Being a Positive Bystander: Male Antiviolence Allies' Experiences of Stepping Up", Journal of Interpersonal Violence, 27(1): 62–83, at 75 (2011)
- "Being in a Room with Like-Minded Men": An Exploratory Study of Men's Participation in a Bystander Intervention Program to Prevent Intimate Partner Violence, The Journal of Men's Studies, 19(1):3–18 (2011)
- Dawgspeak: The Slanguage Dictionary of The University of Georgia, Retrieved May 29, 2012 (citing 1972 E. A. FOLB Compar. Study Urban Black Argot Mar. 135 "Cock block, to interfere with a male's attempt to ‘win over’ a female.")