Green’s Dictionary of Slang

mustn’t-mention-’ems n.

trousers.

1853		N.Y. Clipper 27 Aug. 2/4: He tore his musn’t-mention-’ems.
1859		Cambridge Chron. 20 Aug. 6/2: Mr Jones and wife, while travelling through the ‘bush’ in haste, met with a melancholy accident [...] recorded by an Adelaide poet [...] ‘And while retreating through the woods [...] He tore his mustn’t-mention-’ems, and had to put on hern’ .
1871		R.H. Newell Orpheus C. Kerr Papers 321: A bull-dog of the kind / That tears your mustn't- mention-' ems.