Green’s Dictionary of Slang

brownie king n.

also b.k.
[brownie n.1 (4a) + play on the passive brownie queen n.]

(US gay) a male homosexual who takes the active role in anal intercourse.

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[US]B. Rodgers Queens’ Vernacular 17: The man who fucks in anal intercourse, as opposed to the one who is fucked [...] brownie king [BK].