A kilt was a knee-length skirt-type garment which was the traditional dress of men and boys in the Scottish Highlands. It was characterised by Zoe Heriot as "a barbaric form of garment as worn by a kiltie." She associated it with Scandinavia, specifically Denmark. (TV: The Wheel in Space)
Scottish Highlander Jamie McCrimmon spent his time travelling with the Second Doctor wearing his kilt, even when it was too cold to do so. (TV: The Highlanders, The Abominable Snowmen, et al) He resented it being described as a skirt. (AUDIO: The Glorious Revolution)
During a trip to Edinburgh in 1645, Nardole wore a kilt complete with a Sporran and Tam O'Shanter before being told to remove them by the Twelfth Doctor. (PROSE: Plague City)
On 19 May 1884, Justice Burrows compared the Judoon Captain Kybo to "a rhinoceros in a kilt". (AUDIO: Judoon in Chains)
Highland soldiers fighting as part of the British Army in World War I wore kilts, leading their German adversaries to call them "the devils in skirts" and "the ladies from hell" in reference to their attire. (AUDIO: The Mouthless Dead) Similarly, a Selachian once mistook Jamie McCrimmon's kilt for a skirt, and therefore believed Jamie to be female. (AUDIO: The Selachian Gambit) Zoe initially mistook Jamie's kilt for a female garment. (TV: The Wheel in Space)
The Brigadier wore a kilt meeting the Fourth Doctor, Sarah Jane Smith and Harry Sullivan in Scotland. He explained to Sarah Jane that the reason he was wearing it was that his family was from the clan Stewart. (TV: Terror of the Zygons)
When the Fifth Doctor was giving Erimem a tour of the TARDIS, he told her the kilt in the wardrobe belonged to Jamie. (AUDIO: No Place Like Home)
Behind the scenes
There was some initial discussion about the possibility of Strax wearing a kilt in the TV episode The Name of the Doctor, as Strax is in Glasgow at the start of that episode. The character would have worn the kilt throughout the instalment. However, the garment would have required Strax actor Dan Starkey to wear prosthetic legs and there was insufficient time for those to be crafted. Starkey remarked about the kilt, "It would have been hell to wear. But it would have been great!" Vastra actress Neve McIntosh agreed that having Strax wear a kilt throughout the episode would have been "fantastic." (DWM 475, p. 18)