A man who looks like a woman with no intention of looking like a woman, specifically in the context of anime. The term stems from anime discussions on 4-chan.
The "trap" refers to a situation in which a woman tells her spouse that she is using birth control pills, when in reality she isn't. It's called the trap because if the woman becomes pregnant "accidentally," the man is essentially trapped for the rest of his life, having to raise the kid and pay child support.
The trap is a serious problem in the urban black community.
For a man, the best defense against the trap is to use a condom every time.
It's the term used for a skank who you have a drunken one night stand with who decides its a brilliant foundation to start a family, and hence tried to trap you.
If ever you need more clarification, listen to Ed - Ucation, from the Chronic 2001, by Dr Dre.
Amy: I'm pregnant and I'm keeping the baby.
Jamie: What you're a trap.
Amy: I want CSA payments, cos its definitely yours.
Jamie: fuck off and die! Trap!