Green’s Dictionary of Slang

ineffables n.

trousers.

1823		W. Cory New Monthly Mag. VIII 337: Our lower garments, or Ineffables, sit but awkwardly.
1846		Eclectic Mag. Sept. 11/1: A coward who had dropped his ‘ineffables’ while running away.
1850		L.Hunt Autobiog. Ch. iii: It was said that [...] the small clothes not being then in existence, and the mutton suppers too luxurious, the eatables were given up for the ineffables [F&H].
1867		W. Cory Letters and Journals (1897) 196: Shoes off, ineffables tucked up.
1887		Bristol Mercury 12 July 2/6: They consisted merely of an old overcoat [...] and a pair of ‘ineffables’.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1942		W. Winchell On Broadway 6 May [synd. col.] Mid-Victorians considered the word ‘trousers’ shocking and referred to them in public as ‘ineffables’.