When you and your families/roomates/girlfriends tooth brush are all in the same cup for holding when not in use and the bristles of your brush are touching the bristles of there brush and therefore the toothbrushes are kissing.

Generally a bad thing because the persons plaque is now shared with your plaque. IT is not a bad thing however when it is with a hot girl.
I got so disguisted when I saw that my toothbrush had done a toothbrush kiss with my fat roomie Kevin's toothbrush.
by danibee June 4, 2006
Get the toothbrush kiss mug.