To complain, from Yiddish 'kvetshn' meaning to squeeze, to complain. Originally 'quetzen,' High German, meaning to squeeze.
Also as a noun for being a complainer.
"You'd better not kvetch, or eat chozzerai, You'd better not greps, I'm telling you why... Yenta Claus in coming to town!"
Ba humbug! Ebenezer is such a "kvetch." He refuses to allow the children a little fun!