TWACK v. To strike with great
force using the back of the middle finger (usually the
nail) after coiling it under pressure against the opposing thumb. Accomplished twackers can inflict great welts upon their
victim's flesh.
That turd
cutter pissed me off, so I twacked him right in the
munt!
On the
first twack
Pope jumped like a macaque;
On the second twack
Pope was taken aback;
On the third twack
Lost his marbles the old hack.
(A. S. Pushkin)