it

   1. the sexual attractiveness of females as perceived by males

   A survival from the 1930s prudery about sex:

    'It is not beauty that makes every head (except one) turn on the beach to look at her.' 'It's IT, my boy,' said the Major. (Christie, 1940)

   2. copulation

   A usage without any prior reference to the subject matter:

    I would have asked you anyway... you see, I like it with you. (Bradbury, 1976)

   3. the male or female genitals

   Again, the subject has not been previously introduced:

    Whereas in Jake's youth he had gawped at a girl with upper clothing disarranged to reveal a, to him, rare glimpse of 'them', he is now horrified to find himself staring much lower down at a sharply focussed full-colour close-up of 'it'. (F. Muir, 1990, writing of K. Amis's Jake's Thing)