Based on the Internet citations below, I have no doubt that "zetz" is a Yiddish word meaning a strike or hit. My Dad would have gotten it from growing up in Brooklyn.
"I said, "Ma! I'll bring back your Torah! Don't yell!"
And then she gave me such a zetz, anyway, on the ear, that she woke me up."
"Nobody could get a zetz from the Sanzer's cane and not leave this world without
fixing what he came here to do. In later years, the Sanzer discontinued using
his cane on the hasidim, until the wedding of his youngest grandaughter.
The hasidim lined up before the Rebbe and each got a zetz from the cane of
the Holy Sanzer."
"To gaze at (Al) Franken's face is to see the schlemiel who's about to give you such a zetz."
"The Jews had everything going for them . . . they were invincible. God was taking care of everything, and now we complain? For complaining under those circumstances, you deserve a “zetz” – you deserve a slap across the face."