Mano cornuta.



Here you go, Monk.



Wikipedia :: Corna (aka "the horns" or "rock on") hand gesture



Related:

The Onion :: Metal Council Convenes To Discuss 'Metal Hand Sign' Abuse



"I remember a time not long ago when the Devil Horns were reserved for only the most righteous of person, deed, or riff," Grand Elder Lemmy Kilmister said. "To see someone throwing the horns to his mate at the launderette because the clothes dryer came to a full stop just as he finished reading his copy of Circus... It breaks my heart.