Green’s Dictionary of Slang

used as euph. substitution for an obscenity.

1932		R. Forbes Red Horizon 205: I had been an unmentionable fool, but through some stroke of good fortune I had come through .
1936		W.J. Blackledge Legion of Marching Madmen 170: An unmentionable mixture was wrapped in leaves. One could smell this home-made brand [of tobacco] several yards away!