moon2 [intransitive and transitive]
to bend over and show your buttocks as a joke or a way of insulting someone
moon about/aroundphrasal verb
British English informal
to spend your time lazily, moving around with no real purpose:
I wish you'd stop mooning about and do something useful!
moon over somebody/somethingphrasal verb
to spend your time thinking about someone that you are in love with:
She sits mooning over his photograph for hours.