It's slang from around the turn of the 20th century. No joke, it comes from birdsong hobbyists, when two birds were "competing" they would insert deliberately

It's slang from around the turn of the 20th century. No joke, it comes from birdsong hobbyists, when two birds were "competing" they would insert deliberately off and loud notes to intimidate and silence each other. They named those "slams" like a door slam. That became the slang for the patter of street hawkers when they negged you or heckled someone else for laughs and attention, and eventually became slang for any kind of insult or put down.