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The word is said to have been coined when Prime Minister of Finland Ahti Karjalainen visited a Kenyan zoo in the 1970s. He is reported as having said "Kaikki eläimet ovat tankeroita" (Finnish for "All animals are tankeros"), after misunderstanding a sign that said "All animals are dangerous."
This was so popular that one Finnish magazine held a drawing competition of what the tankero would look like. The Finnish political cartoonist Kari Suomalainen drew a strip where Kalevi Sorsa meets Karjalainen and a strange-looking animal he is carrying on a leash. "Is that a tankero?" asks Sorsa, and the strange-looking animal replies "No, that's Ahti."
During the same period a certain number of jokes, based on English–Finnish language misunderstandings, were current: for example a person mistakenly greeting a Crimean by calling him criminal. These jokes were labelled as "tankero jokes" and generally told with Ahti Karjalainen as the main character.
The original meaning fell into disuse after Karjalainen's death, and today it is a slang word for something that is awkward or not properly developed, especially poor knowledge of the pronunciation or grammar of a foreign language.
The most common use of the word tankero today is "tankero-englanti" or "tankero English", meaning poorly pronounced English. The term has very strong connotations of awkward-sounding pronunciation with a very strong Finnish accent. The actual grammar and vocabulary of "tankero English" may well be correct, though.