In live theater, or more specifically in stage combat, the word knap (silent k) is used for the sound of a blow landing. Usually, that sound is made by something other than what the audience sees as the contact— such as actor being “slapped” without contact being made hitting their upstage thigh with their own hand, or an actor throwing a punch hitting his hand instead of the “target”. An offstage actor can provide the knap, although that is more difficult to time. Sometimes in stage combat actual contact is made, in a way that is not dangerous or painful but provides a loud knap (sometimes on armor).
This is one of the theater terms that I don’t think is widely known outside the theater world—I have the impression that the thing itself is pretty well-known, but not the word for it.
As for its derivation, it’s probably just the sound, but there was Middle English word knap, which as a verb meant to strike something sharply and as a noun meant either the blow or the sound of a blow. It’s listed in the OED as ‘dialect’ or ‘obsolete’ but also in frequency band 4, which means that I think the frequency is probably picking up the theater usage, despite that not making it in to the entry itself.