From The Collaborative International Dictionary of English v.0.48:
Gnar \Gnar\, n. [OE. knarre, gnarre, akin to OD. knor, G.
knorren. Cf. Knar, Knur, Gnarl.]
A knot or gnarl in wood; hence, a tough, thickset man; --
written also gnarr. [Archaic]
He was . . . a thick gnarre. --Chaucer.