Proper noun; a person who is bullshitting, but with great skill and humour and sounds like they might be telling the truth, but something indicates a lie based on the entire story in context.
However a perfectly valid story can earn the teller the appellation Muggy in order to dis them or cause general laughter in order to change to a lighter mood. Calling Muggy usually means the caller has to explain why or bullshit in return. Friendly arguing ensues with increasingly clever insults.
He said (with great earnestness) I sat there after the party and damned if I didn't look up just in time to see a strange object go across the sky. I'm not the only one who saw it, so and so were there too!
Friend says: Muggy (need to draw it out with drawl Mawwwwwgeeeee) I wasn't even there, and you were drunk out of your tree.