Wikipedia
Longcloth refers to a plain cotton cloth originally made in comparatively long pieces.
The name was applied particularly to cloth made in India. Longcloth, which is now commonly bleached, comprehends a number of various qualities. It is heavier than cambric, and finer than medium or Mexican. In the early 1900s, as it was used principally for underclothing and shirts, most of the longcloth sold in Great Britain passed through the hands of the shirt and underclothing manufacturers, who sold it to the shopkeepers, though there was still a considerable if decreasing retail trade in piece-goods. In the UK in the early 20th century the lower kinds of longcloth, which were made from American cotton, corresponded in quality to the better kinds of shirting made for the East, but the best longcloths were made from Egyptian cotton, and were fine and fairly costly goods.
Nowadays, longcloth (or long cloth) designates a cotton fabric which is of high quality, very soft, coarsely woven, and very often used to make underwear and infants' clothing.