The English word "pajamas" (or "pyjamas," in British spelling) is a borrowing from Hindi/Urdu, which itself borrowed it from Persian. The Persian compound is pāy ("leg") plus jāma ("garment") — a "leg garment," meaning loose-fitting trousers held up by a drawstring at the waist.
Trousers, Not Sleepwear
In Persia, the Mughal Empire, and across much of Central Asia, pāy-jāma were everyday outdoor clothing worn by both men and women. They were not associated with sleep; people wore them in the street, in the bazaar, at work. The British, encountering them in colonial India, found them comfortable for the climate and adopted them as informal warm-weather lounging clothes.
Into the Western Bedroom
British colonial officers, returning home with their adopted habits, popularised pajamas as nightwear in the late 19th century. They were marketed in London as a healthy alternative to the long nightshirt that had been the standard male sleepwear for centuries. By the 1880s, "pyjama suits" — matching jacket and trousers, often in striped cotton — were a fashionable item in British outfitters. Women's pajamas became fashionable in the West a generation later, in the 1920s.
The Spelling Split
The split between British "pyjamas" and American "pajamas" follows the broader pattern of British vs. American English spelling preferences. The Anglo-Indian community originally used both forms; American writers gradually converged on the cleaner "pajamas," while British writers retained the more Persian-looking "pyjamas." Either spelling is etymologically defensible, since the original Persian word can be Romanised either way.