From this week's "Talk to The Times" web Q&A with crossword puzzle editor Will Shortz:
It's true that the word URINE has never appeared in a New York Times crossword — or any other crossword I'm aware of. Margaret Farrar, The Times's first crossword editor (1942-69), followed the philosophy of "good news only," not allowing unpleasant and impolite language, and this rule still holds today. As Merl Reagle, crossword constructor extraordinaire, explained in "Wordplay": "They're sitting there relaxing ... and here comes RECTAL? I don't think so."