Originally;
Welsher, to. (v.) - the act being completely inept in social situations, in such a way that makes you appear to be a complete ass. (i.e., Matt Welsher.)
As in, making fun of someone's face while they are, unbeknown-st to you, standing right behind you, or calling someone something totally, horribly offensive without even realizing that you did so.
Elyse!
(That's me.)