Someone who lies about their activities with a member of the opposite sex to try and look cool, gain friends approval, make conversation, basically exaggerates the situation.
guy 1 = "did you take that bird home last night"?
guy 2 = "yea sure did, ploughed her everywhere, did her up the arse, AND i even fucked her by the window"
guy 1 = "wow, nice one lid"
guy 3 = "no you didn't, i know her, she said you tried to kiss her, she fell asleep, you threw up everywhere then walked home, you're a total Sex Liar"