I stumbled upon this definition earlier today. Not the best I have ever read.

Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.

kilt, kilts (nn.)

Kilt is the standard singular form of the name of the Scottish man’s skirt, although, on analogy with pants and trousers, it is frequently heard from Americans as kilts even when modified by a singular his. One bagpiper wears his, a, or the kilt, not kilts; the members of a pipe band wear kilts (or the kilt). Scottish-Americans usually insist on kilt, not kilts; courtesy suggests that you do the same.

or

The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.

kilt

PRONUNCIATION: klt
NOUN: 1. A knee-length skirt with deep pleats, usually of a tartan wool, worn as part of the dress for men in the Scottish Highlands. 2. A similar skirt worn by women, girls, and boys.
TRANSITIVE VERB: Inflected forms: kilt·ed, kilt·ing, kilts
To tuck up (something) around the body.
ETYMOLOGY: From kilt, to tuck up, from Middle English kilten, of Scandinavian origin.

I'm not really pleased with either defintion. The American Heritage defintion of skirt includes the line 2. A garment hanging from the waist and worn by women and girls.
Some things never change!