why call it "The Kilt"?

[Irish Jack O'Brian]

Gaelic carryover for sure. The same usage of 'the' can be found all through Irish and Highland english as written and spoken.

And I wonder if it might not also be related to the Auld Alliance, since the French speak often of "the" this and "the" that where English would say "a" or use no qualifier...for example one combs les cheveux - the hair, etc.