Green’s Dictionary of Slang

extremely tight trousers.

1828		Lytton Pelham II 107: He only filched a twopenny-halfpenny gilt chain out of his master, Levi, the pawnbroker’s window, and stuck it in his eel-skin to make a show.
1884		H. Smart Post to Finish II 249: A gentleman [...] whose nether garments fitted his extremities like eelskins.