Derived from the root word "Twunt" an amalgamation of "Twat" and "Cunt." Twuntish is a slightly more polite way of saying "twattish" or "cuntish" behaviour.
Crass behaviour to the point of rudeness.
Unfriendly, crass, stupid, nasty, rude, insensitive or spiteful behaviour.
That was a rather twuntish thing to do.