A gender non-conforming bisexual woman. Kind of like a butch lesbian, but bisexual. May have short hair, wear men's or men's-style clothes, be unshaven, or any or none of the above.
Girl 1: Hey, check out that cute girl over there in the basketball shorts. Is she gay?

Girl 2: No, she swings both ways. She's a total tom-bi.
by TheTomcat May 7, 2016
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