Another term for a lesbian. To get a visual example, bend both the index and middle finger of each hand and bump the mid knuckles together thus simulating two vaginas bumping each other as lesbians do to get sexual pleasure, such as the extremely hot act of tribbing.
Guy 1: "Hey dude, have you met the new girl? She's hot!"
Guy 2: "Yeah, but dude she's a knuckle bumper. There's rainbows and shit all over her social network profile."
Guy 2: "Yeah, but dude she's a knuckle bumper. There's rainbows and shit all over her social network profile."
by MrFriendzone January 2, 2016
by daddy2272727 May 19, 2017