Past tense of gnar; grew in a knotty, twisted way or made a snarling sound.
From Old Norse 'knarr' meaning knot or protruding knob. The word evolved in English to describe both the physical twisting of wood and the aggressive snarling sound made by animals.
Gnarled wood is actually stronger in some ways because the knots represent where branches fell off and the tree healed itself—so a gnarred tree is a survivor!
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