I had thought long shirts to have been worn in medieval times (in Western Europe at least) as a form of undergarment, with the long back of the garment pulled forward and then the braies / hosen pulled up to keep things in place.
Iām not entirely sure how this worked with a plaid / great kilt (what with no hose and all)ā¦ but understand that that the shirt ends may have been knotted to form a breech clout of sorts.