racist – A statement of surrender during an argument. When two people or disputants are engaged in an acrimonious debate, the side that first says “Racist!” has conceded defeat. Synonymous with saying “Resign” during a chess game, or “Uncle” during a schoolyard fight. Originally, the term was meant to indicate that one side was accusing the other of being racist, but once it was noticed that people only resorted to this tactic when all other arguments had been exhausted, it acquired its new meaning of “indicating one’s own concession of defeat.”
Source: Charles Johnson, from the late and much lamented Little Green Footballs Dictionary.