Flannel
Middle English: probably from Welsh gwlanen ‘woollen article’, from gwlân ‘wool’.
wiktionary
From Middle English flaunneol, from Anglo-Norman flanelle (compare Norman flianné), diminutive of Old French flaine, floene(“coarse wool”), from Gaulish, from Proto-Celtic *wlānos, *wlanā(“wool”) (compare Welsh gwlân, Breton gloan), from Proto-Indo-European *h₂wĺ̥h₁neh₂. More at wool.
etymonline
flannel (n.)
"warm, loosely woven woolen stuff," c. 1300, flaunneol, probably related to Middle English flanen "sackcloth" (c. 1400); by Skeat and others traced to Welsh gwlanen "woolen cloth," from gwlan "wool," from Celtic *wlana, from PIE *wele- (1) "wool" (see 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].
The Welsh origin is not a universally accepted etymology, due to the sound changes involved; Barnhart, Gamillscheg, Diez suggest the English word is from an Anglo-French diminutive of Old French flaine "a kind of coarse wool." Modern French flanelle is a 17c. borrowing from English.
