Mr. Bartlett, in the fourth edition (1877) of his Dictionary of Americanisms, decided that the word “Dingbat” owed its origin to a bat, or piece of wood or metal, that could be dinged, or thrown.
Well, maybe so.
That’s probably as good as any explanation, though the source of any bit of slang is, usually, highly dubious.
At any rate, we Americans use it, as we do its derivative, dingus, as a momentary name of anything of which the proper name is out of mind or unknown. (See also thingum.)