Green’s Dictionary of Slang

1. absolutely furious.

1902		Bulletin (Sydney) 22 Nov. 13/4: The chess clubs are the latest crowd to kick against paying their just telegraphic dues. [...] Now that the Dept. ukases that all must come in at the same rate, some of the chess cranks foam at the mouth.
1912		Bulletin (Sydney) 3 Oct. 48/2: There he found the Inspector foaming at the mouth over the disappearance of his horse.
1978		L. Kramer Faggots 119: Dearie me, whatever is Blaze foaming on at the mouth about so!
1999		E. Bunker Mr Blue 351: Reading Dostoesvsky was like listening to someone foaming and overwrought as they told stories about the souls of human beings.

2. (US) of a penis, on the verge of ejaculation.

c.1960		in E. Cray Erotic Muse (1992) 356: So stroke! stroke! you mater-Betas! / Raise your foaming cocks on high.
1966		Trimble 5000 Adult Sex Words and Phrases 80: foaming at the mouth (Vulg.) On the verge of Ejaculation when a fore-discharge of fluid emits from the Meatus.
1980		Maledicta IV:2 (Winter) 196: When the red hot poker is foaming at the mouth and ready to spit, the gasp and grunt (rhyming slang) should be ready to let down its spendings.