shortened version of 'drainpipes' ie very tight trousers.
the use of 'drainp' was originally founded by ashley and aziz, who have recently developed a language totally devoted to shortening any word...if it sounds right.
for example:
drainpipes = drainp
guildford = guilf
house of fraser = house of frase
positive tension (by bloc party) = pos tensh (by bloc part)
arctic monkeys = ark monk
banquet = banq

etc etc etc....