According to the Online Etymological Dictionary, a slapstick was "a device consisting of two sticks fastened together so as to slap loudly when a clown or actor hits somebody with it, or to make a sound-effect offstage."
According to citations in the Oxford English Dictionary, it was also used to
intimate intimidate children. From a 1907 story in a weekly magazine: "The special officer in the gallery, armed with a ‘slap-stick’, the customary weapon in American theatre galleries, made himself very officious amongst the small boys."
The most intriguing OED citation comes from a 1950 newspaper article: "The 50-year-old clown... said that when he bent over another funnyman accidentally hit him with the wrong side of a slap-stick. He explained that a slap-stick contains a blank ·38-caliber cartridge on one side to make a bang."
The whole article, titled "Clown's Pants Catch on Fire" is reproduced here.