It means to french kiss with someone. It falls from french kiss to french to french's which is a popular brand of mustard.
Friend: Did you get to mustard today?
Friend of: Nope
Friend: Why?
Friend of: too many peoeple around
Friend of: Nope
Friend: Why?
Friend of: too many peoeple around
by reginaismynameurstoryismygame July 22, 2008