gnarl (v.)

"contort, twist, make knotty," 1814, a back-formation from gnarled (q.v.). As a noun from 1824, "a knotty growth on wood." Earlier an identical verb was used imitatively in a sense of "to snarl" like a dog (1590s); Farmer and Henley ("Slang and Its Analogues") lists gnarler as thieves' slang for "a watch-dog."