When a girl, intentionally or unintentionally, causes a nearby dude to acquire a boner. It usually happens accidentally when the girl assumes a position that either reveals skin or can be construed as sexual, such as bending over in a low-cut shirt and giving a guy a better view of her breasts, and/or reminding him of doggy style. This can also happen with other positions, sometimes assumed in gymnastics or other activities.
Chicks can also do a variety things intentionally to accomplish the same thing, anything from winking to straight up reaching down a dude's pants.
Also can describe the state of having a boner as a result of one of these occurences.
Courtney dropped some change and bent over to pick it up, giving me a view of her tits, and unwittingly bebonered me, so I sat down to conceal my erection and deal with it accordingly.
My girlfriend whispered in my ear that she was wet, and I became bebonered and remained in that state until after sex.