The kilt is an item of traditional Scottish Highland dress. Nowadays most Scotsmen see it as formal dress. It is generally worn only at weddings or other formal occasions, although there are still a few people who wear it daily.
After the unrest of the 18th century, the kilt, along with other features of Gaelic culture became identified with the Jacobites. As a result the Dress Act of 1747 made it illegal to wear the kilt in Scotland; the law was repealed in 1783. An exception was made in the years following 1747 to allow the kilt to be worn in the military -- made to try to increase recruitment into the army and placate the Highlanders at a time when the British government could ill afford another civil war with the Highlands of Scotland.
The popularity of the current garment is largely due to the Highland romantic revival orchestrated by Sir Walter Scott for the visit of George IV to Edinburgh at the beginning of the 19th century. The modern kilt is often confused with the Leine Croich, a knee-length shirt of leather, linen or canvas, heavily pleated and sometimes quilted as protection. However it is really more like the Feileadh Beag, or little kilt, made with half the width of material of the 'great kilt', designed for military wear and so made of tartan or tweed cloth, box-pleated or knife-pleated (with pleats sewn in to speed the donning of the kilt) with the lower edges reaching not lower than the centre of the knee-cap. It dates from around the mid-18th Century, replacing the simpler belted plaid. The kilt is traditionally for men only, although in the modern era, long women's dresses patterned after kilts do exist, and women pipers frequently wear kilts. Kilten skirts for girls are also worn.
Traditionally, what a Scotsman wears under his kilt is his own business and generally, Scotsmen will be at pains to keep it so. Thus any reply to such a question may hint at the answer but should never state it outright. A good standard reply when asked, is that, "Nothing is worn under the kilt. It's all in perfect working order".