First and foremost this word describes someone who is trying to be "put on, be chosen," or simply garner lots of attention. If you don't know what those things mean, chances are you're using the word "thristy" incorrectly.
Examples of people who define "thirsty":
1.That chick at the club trying desperately to get into the VIP section.
2.The guy that hits on every last chick in a group of girlfriends.
3.The girl or guy whose top goal is to impress other people (this could be dressing a certain way solely to obtain complements, frequenting certain establishments just to be seen there, etc.)
If you have friends who fit this description it is your duty to call them out. If you find yourself fitting into one of the aforementioned categories, do some soul searching, IMMEDIATELY! If you see someone you don't know or care about who fits these descriptions, just sit back and enjoy the live entertainment.