Gnarly

Gnarly \NAHR-lee\ , adjective;
1. Gnarled
2. Slang: Distasteful; distressing; offensive; gross

Methuselah, gnarly (source)

This quintessential 80's teen slang word actually dates to the mid-1800's, though the slang meaning really only dates to the 1970's as surfer slang. Gnarly meaning "gnarled" is first attested in 1846 and is based on the noun gnarl, which is a back-formation of the adjective gnarled. Gnarled was used by the almighty Shakespeare himself and is a variation on knurled from Middle English knurl ("a rock, a stone"), which is of uncertain origin.