trapdar
Similar to how a gaydar is the ability to detect non-heterosexuality, the trapdar is the ability to detect a trap--that is, a male who can be easily mistaken for a female.
<A> That chick's pretty hot, I should ask her out.
<B> Don't. That "chick" is setting off my trapdar.
<B> Don't. That "chick" is setting off my trapdar.
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