The slang term "the clap" for gonorrhea is believed to have originated from the French word "clapier," meaning "rabbit hutch" or "brothel," where such diseases were often contracted.
Here's a more detailed explanation:
"Clapier" in French:
In the 1500s, "clapier" referred to small huts where prostitutes often lived and serviced their customers, and later, to the brothels themselves.
"Clapier bubo":
In French, the disease became known as "clapier bubo" (or clapior bubo), with "bubo" referring to an inflamed lymph node or infection in the groin area.
Shortened to "the clap":
The English likely shortened "clapier bubo" to "the clap".
Other theories:
Some theories suggest the term might come from an old English word "clappan," meaning "to beat or throb," or from an (un-evidenced) medieval practice of clapping one's genitalia to reduce pain.