A crossbreed between two of the most popular words in the Teenage Vocabulary; Gnarly and Beasty. Gnarbeasty refers to someone, something, or some action that is (one or more of the following) cool, awesome, sweet, sick, wicked, badass, dank etc. AND something that (one or more of the following) is big/bulky, hot/sexy, amazing, and other forms of the word cool and or big.
"There was a Gnarbeasty spider on my wall the other day." (The spider was Gnarly in appearance, but big and Beasty)
"We were boarding on chair 6 and I caught some Gnarbeasty air off the kicker we built!" (Basically, I caught a WHOLE LOT of air in a gnar fashion)