Green’s Dictionary of Slang

bluff n.2

[butch n.1 (6) + fluff n.1 (6)]

a female homosexual who can alternate between active/passive roles.

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[US]J.P. Stanley ‘Homosexual Sl.’ in AS XLV:1/2 54: Female homosexuals listed bluff, [...] a cross between butch and fluff, and is used to refer to an individual who plays either the aggressive or the passive role.