Jazzgirl, Chambers gives "trews" as tartan trousers, and many Scots would pronounce trousers as "troosers"!
So maybe that is where the word is coming from?
I think it was in the 60's that the late Andy Stewart had a hit record (certainly up here) with a song called "Donald, Where's Your Troosers?"
About a young lad from Skye venturing to the big city in the kilt!
I did try to find out more about trews, and to that end, had a wee look at Anne Bradford's "Underwear", if you know what I mean.
No sign of "trews", but she has "butt bra"?
Am I too young to know what a butt bra is?
PS. Andy Stewart used to do a great impersonation of Elvis, too!