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c.1500, probably from Welsh gwlanen "woolen cloth," from gwlan "wool," from Celtic *wlana, from PIE *wele- "wool."
The Welsh origin is not a universally accepted etymology, due to the sound changes involved; some (Barnhart, Gamillscheg) suggest the English word is from an Anglo-French diminutive of Old French flaine "a kind of coarse wool." "As flannel was already in the 16th c. a well-known production of Wales, a Welsh origin for the word seems antecedently likely" [OED]. Modern French flanelle is a 17c. borrowing from English.
fabric made in plain or twill weave, usually with carded yarns. It is napped, most often on both sides, the degree of napping ranging from slight to so heavy that the twill weave is obscured. Fibre composition and amount of napping are dependent on the intended use. Flannel is a relatively warm fabric, since still air is held in the fabric because of the napping. Addition of a man-made fibre to the blend increases the resistance to abrasion and hence may lengthen the life of the fabric. Furthermore, some of these blends help to prevent stretching, so that a better fit is maintained. Crease retention is improved with some blends such as acrylic fibre.