"cloth from woven flax," early 14c., noun use of adjective linen "made of flax" from Old English līn "flax, linen thread, linen cloth" + -en (2). Old English lin is from Proto-Germanic *linam (source also of Old Saxon, Old Norse, Old High German lin "flax, linen," German Leinen "linen," Gothic lein "linen cloth"), probably an early borrowing from Latin linum "flax, linen," which, along with Greek linon is from a non-Indo-European language. Beekes writes, "Original identity is possible, however, since the cultivation of flax in Central Europe is very old. Still, it is more probable that linon and linum derive from a Mediterranean word. The word is unknown in Indo-Iranian (but the concept is, of course)." Lithuanian linai, Old Church Slavonic linu, Irish lin probably are ultimately from Latin or Greek.
Woolen has begun the same evolution. Meaning "articles of linen fabric collectively" is from 1748, now sometimes extended unetymologically to cotton and artificial fabrics. The Old English noun also carried into Middle English as lin (n.) "linen" and persisted into 17c. and later in technical uses. The Middle English phrase under line (c. 1300) meant "in one's clothes." Linen-lifter (1650s) was old slang for an adulterous male.