Adjective, also a verb. To be "over the
moon" about something. Whether you are
brown-nosing a person you respect, obsessing over your crush or just being another
lame fan. Even applicable to those cocky enough to be moony over themselves. The key component is that you have to embarass yourself in some way through your starry eyed eagerness.
"I wanted to show up at his
work to surprise him, but I thought that would be too moony"
"She was so
busy mooning over her professor she didn't
even realize class was over"