Green’s Dictionary of Slang

very narrow ‘drainpipe’ trousers.

1977		Eve. Standard 17 Aug. 13: The baggy trousers he wore had nothing on the drainies of the local Teds.
1998		K. Sampson Awaydays 10: Shit Harrington, shit, nondescript Wrangler drainies, shit, dirty Green Flash.