The Rice University Neologisms Database

Anything challenging, difficult, or physically rough. The formation of the word gnar shows that deriving nouns from adjectives and verbs is not a standardized or reversible process. Gnar is a noun derived from the adjective gnarly, which is further derived from the noun gnarl, a late form of the middle English word knar, which refers to a knot in wood. In this process, which was catalyzed by surfer slang and those who used it, gnarl lost its final l and came to refer to something broader. Waves breaking on sharp rocks would be described as gnar, as would an intense wound, e.g. “Look at all that gnar on his arm from when he tried to break his fall.”