Green’s Dictionary of Slang

slacks n.

In phrases

haul up one’s slacks (v.)

to enthuse, to opinionate.

[UK]Wodehouse Young Men in Spats 22: Then, with the cotelettes and mashed, Mavis started to haul up her slacks about the Soul of America.
scare the slacks off (v.)

see scare the pants off under pants n.