Moon over
Definition 1 of 1
Pro Tip 1/3Emotional Focus
The phrase suggests an emotional preoccupation, often implying a lack of attention to other matters. 
She spent the entire class mooning over her vacation photos, ignoring the lesson.
Pro Tip 2/3Informal Speech
'Moon over' is informal and usually used in casual conversation rather than formal writing. 
Mike's been mooning over his new guitar all week.
Pro Tip 3/3Negative Connotation
The phrase can imply that the attention given is excessive or unproductive, and sometimes foolish. 
He's mooning over a celebrity who doesn't even know he exists.