Good-looking person, usually dressed and/or posing provocatively. Often includes engagement-bait-type caption.

Someone is "thirsty" for sex. A woman (or a broad approximation there of) exploits that thirst, hence "trap".

The "woman" is the thirst trap. Woman is in quotes, because it's almost always not a woman, but a bot, or one of many accounts run by scammers.

A sexually suggestive picture

Where lions ambush gazelles when they go to the water hole

No, only other groups are stupid, but not my group! /s