As in, one who attempts to ingratiate himself to a clique of attractive young women, in the hopes that he will copulate.
Then promptly moves to another clique when rejected.
Steve: nothing man... i heard a strange noise and thought i would just check it out....
George: i could see your feet under the door and could hear you breathing asshole!! your such a sniffer!!!
Sniffers can be commonly noticed at parties, events, family functions, in the work environment, and most social gatherings.