c.1600, probably a variant of knurled, from Middle English knar "knob, knot in wood" (late 14c.), earlier "a crag, twisted rock" (early 13c.), from a general group of Germanic words that includes English knob, knock, knuckle, knoll, knurl. Gnarl, v. "make knotty," gnarl (n.) "a knotty growth on wood," and gnarly, adj. all seem to owe their existence in modern English to Shakespeare's use of gnarled in 1603:"(Gnarled) occurs in one passage of Shakes. (for which the sole authority is the folio of 1623), whence it came into general use in the nineteenth century" [OED].