Male underwear often sported by non-lipstick-lesbians.
Knickerbockers is an old-school name for panties/underwear worn by the fairer sex. Butch vagitarians sometimes prefer male clothing and will wear men's boxer shorts in place of panties. Hence, 'knickerboxers'.

Craig: Hey man, do you sell boxer shorts?
Jerome: Yeah, over there by the butch chick.
Craig: What's a rugmuncher doing buying shorts?
Jerome: She wears em as knickerboxers, Dude.
by Conrad Fado October 7, 2011
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