a way to describe the degree of sexual arousal as a result of a given sight, situation, or circumstance. Usually used as a way to describe a very high level of sexual arousal.
The Hornacity level in the room hit a high point when the speaker took her top off and shook her tits at the previously bored audience. Now awakened, the audience licked their chops with hornacious desire.
Ashton: How's the sex life?
John: Well there's this one bitch Mary I've been talkin' to...
Ashton: No not Mary! She's got the highest hopacity north of the Mason-Dixon.
John: Well than I'm money.
Laura: You're gross.