verb, transitive. To slap another's face with one's penis. Usually said in response to an insult or other cutting remark directed at the speaker.
noun. A slap to the face with the penis.
Note: A molly whop works best when accompanied by the insinuation that the "whopper" has a particularly large penis. This can be accomplished by adding a qualifier after the phrase, or by prior knowledge of the "whoppee" of the "whopper"'s enormous penis.
Jane: Can we please go to the mall today?
John: Jane, so help me god, if you ask me that one more time I'll molly whop ya!
Jack: So I heard you and Jane had a fight the other day. What was all that about?
John: Well, she was being a bitch, so I gave her a good ol'-fashioned molly whop with my burgeoning man-meat.
Jack: Ha ha! That'll teach her!