1) one who behaves like an uncontrolled sexual maniac in public. 2) one who makes sexual advances to strangers in a public setting, with little to no preamble such as asking their name.
(from a quote from the movie HACKERS: "God gave men brains so they wouldn't hump women's legs at cocktail parties.")
1) "That guy's such a leghumper, he really should learn to stop groping everyone, touching himself, and making moaning noises."
2) "Watch out for Stanley; he's a real leghumper whenever he sees anyone halfway attractive."