A guy who will come up to you or even watch you from afar (normally at parties) even when you're with all your friends, and will claim to see you everywhereknows everything about you and tries to hit on you-- when apparently you have no idea who he even is or have maybe only seen him once. Also may be a guy that is trying extremely hard to get in your pants and thinks that you like it--normally these type of guys are very unattractive and are huge losers--commonly found in fraternities.
"Omg did you see that guy? He was creeping on you so bad!"
" Johnny is such a creeper- stay away from that kid)