[A.A. Cunningham]

I'm not Roman Catolic, what is Thwack?

Thwack is a secular term that Roman Catholics, Roman Catolics(sic), Jews, Protestants, agnostics, atheists,et al are at liberty to use.

thwack

thwack (thwàk) verb, transitive
thwacked, thwacking, thwacks
To strike or hit with a flat object; whack.

noun
A hard blow with a flat object; a whack.

[Imitative.]

[LadyPilgrim]

Never heard that term used. & ;-/