- at it
- engaged in some taboo activityIn appropriate circumstances, the phrase can apply to anything from picking your nose to bestiality. In the East End of London, it usually refers to being a villain; elsewhere is may indicate sexual activity:At least one of his uncles is 'at it', as they say, and drives around in a silver-grey Mercedes. (Read, 1979, of a habitual thief) Shit, for all he knew they could have been at it in Paris right from the beginning. (Winton, 1994, of homosexuality)
How not to say what you mean: A dictionary of euphemisms. R. W. Holder. 2014.