A lot of these other definitions are wrong, so here's the actual definition.
(v.) The process by which the sex of a sexually dimorphic animal is determined. This is commonly done biologists and agricultural workers.
Bob: Are you done sexing those animals like I told you to?
Jeff: Yep! It was a bit difficult getting my dick inside that duck, but-
Bob: WHAT THE ACTUAL FU- Sexing means to determine whether the animal is male or female! Are you even a real scientist?!
Jeff (thinking): Ack! He's on to me!
(Jeff then runs away)
another type of "air humping" except you are on the floor grinding. this usually occurs when listening to hardcore songs that involve some kind of sex move.
i saw jake sexing the air when listening to Big Bang.