Definition:: A woman who is well past her prime (usually mid forties and up, though age does not have to be the determining factor) yet is still actively seeking sexual relationships with men who are much younger or well out of her league.
Alternatively a younger women that looks much older, or appears to have been around the block more than her share of times can be referred to as a gynosaur.
Whereas the being, or sleeping with a cougar has become more acceptable in recent years, the same cannot be said of a gynosaur. Gynosaurs are most often cougars that never learned when to stop.
Guy 1: Dude did you see that milf over there?
Guy 2: No, where?
Guy 1: She's standing right over there next to those two gynosaurs.
Guy 1: Hey did I see you mom at the bar last night?
Guy 2: Yeah she was celebrating her 50th birthday with some friends.
Guy 1: Dude she kept trying to hit on me, thats gross she's a total gynosaur.