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A straight male in the midst of a region generally populated by gay men. Said straight male usually plays up his heterosexuality to make it apparent to others/outsiders that he is not gay, sometimes resulting in bro-like behavior.
A: "The waiter at that restaurant on Castro Street was totally a Castbro. He did not set off my gaydar at all."

B: "It's too bad - he's kind of cute."
by thirtytwo March 17, 2011
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