at it

   engaged in some taboo activity

   In 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)