@patiodog,
I don't have a great deal written about the origin of the phrase "the clap", in my possession, but I double checked what books I have at hand, and they agreed with the Fr. def, being the origional source......and that is of course not the only theory, I've read some that go into great detail, in a long round-a-bout way to derive the origin...that is a common thing, especially with slang.....but the simple Fr. def is probably the most accurate.
I have a decent collection of word origin books...etymology...but the ones that are slanted toward vulgar or slang words, tend to be the ones that everyone wants to borrow. {I have a few out on loan at the moment} I have one called "The Slang of Sin", which is a neat little book, and always grabs attention....what it does is it takes a word or phrase, such as "the clap", and then gives multiple examples...such as....
garden gout
cold in the dong
cupid's itch
forgot-me-not
torch of Venus
that sort of thing. There is a lengthy section on masturbation that I can not read without giggling out loud...some of the expressions are very clever, and just funny as hell.