Person who lacks energy/enthusiasm. One who is boring or lame and unwilling to participate in a fun activity because they are too busy being a herb.
I asked Jeremy to come out to the bar with me tonight, but instead he’s going to be a mopey dick and play WOW all night. (world of war craft)
mopey dick: define #2
Ahab tried to get it up so he could bum Queequeg, but there was nothing he could do about his Mopey Dick.