The opposite of an aphrodisiac. Something that turns you off really bad, like bonerkill. You may be having a raging hard-on in class daydreaming about Scarlet Johanssen's titties, and then you see the fat asian chick in front of you scratch her ass and smell her finger. Eww, anti-disiac.
Also, may be something that you think about to get your boner to go down, or stop you from having sex.
(Austin Powers being felt up by fembots): Cold showers... MARGARET THATCHER NAKED ON A COLD DAY!! <==That's a good anti-disiac.
Megan: I just talked to Nooch, and I think I want to be a lesbian now. He's such an anti-disiac.