Comes from the word mope. Someone who is given to chronic fretting, worrying and whining. Found under beds, inside bathrooms and in front of microphones, shouting their depression hoarse.
Similar to, but less abrasive than, saying "Hurry it Up!"; Asking someone, especially in line in front of you, to hustle and finish what they're doing.
I was at 7-11 grabbing a soda before work and the guy in front of me was taking forever buying scratch tickets. After a few minutes I had to say "C'monpal You're Mopin!"
Someone you have sex with that is not only unattractive or obese but smells or is sweaty. A fun ride in the sack nonetheless, that is until your friends find out.
'Oh gross that dirty moped smells. I would still give it to her though, but no one must ever know of this"