AGH! The buttninja are attacking! COVER YOUR ASS!
one who is a sneaky a-hole & hard to spot before it's too late, yet considerably annoying on every level that no one wants to be around...usually due to them just being an idiot & not being able to help their own stupidity..
"That guy who showed up at the last minute to go out with the rest of us is such a buttninja!...we HAVE to ditch him!!