Green’s Dictionary of Slang

what’s-their-names n.¹

a pair of trousers.

1866		Letters by an Odd Boy 7: I should bone a couple of pounds of sauages [...] and stow them away in the pockets of my what’s-their-names.
1881		Bulletin (Sydney) 23 Apr. 8/4: A lady standing 6 feet 6 in her ‘what’s-their-names’ .