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

That's the Lead Poisioned and Proud of it! unit.

Yes, but the line of thinking originates there, and they were programmed to be more hateful and stupid than they ever would have been in a pre-zuck era.