Green’s Dictionary of Slang

French n.

1. [late 16C+] syphilis.

2. [18C–19C] brandy.

3. [late 19C+] taboo language.

4. [1910s+] (also French art, full French) fellatio (cf. Frenching n.).

5. [1970s] pornography.

In phrases

excuse my French (also excuse the French, pardon my French)

[late 19C+] a genteel euph. automatically offered after the speaker has sworn in public.

learn French (v.)

[17C] to become infected with syphilis.

speak French (v.)

[20C+] to indulge in unconventional sexual play.

talk French (v.)

[1940s] (US) to fellate.