Definition of gnarly - Merriam-Webster Online Dictionary

For those who might be confused about the exact meaning of the term “gnarly”—like I was—that is probably because it can mean both something positive or something negative.

The Merriam-Webster dictionary defines gnarly as:

1 : GNARLED
2 slang—used as a generalized term of approval or disparagement

And this American English to British English translator gives this definition:

gnarly /nahlee/ adj :
unpleasant, syn. bogus, gross, but also of something imposing a great challenge, “gnarly waves, dude!”.