gnarly

Etymology

gnarl (“knot in wood”) + -y. The slang senses were particularly popularized by US surf culture in the 1970s.

adj

1. Having or characterized by gnarls; gnarled.
   Out of one window I can see the garden, those mysterious deep-shaded arbors, the riotous old-fashioned flowers, and bushes and gnarly trees. 1892 January, Charlotte Perkins Gilman, The Yellow Wall Paper, Boston, Mass.: Small, Maynard & Company, published 1901, →OCLC, page 13
2. (slang) Excellent; attractive.
   There ain't nothing gnarlier (apparently) than slapping on some brightly coloured sunblock to ward off the blinding spectre of dangerous, snow-reflected sunlight. 16 January 2000, Sunday Herald, Glasgow
3. (slang, US) Dangerous; difficult.
   a gnarly problem
   When the swell struck, the North Shore got gnarly, and the wise ones hit the outer islands where the energy was just as juicy but a bit more organized. 1977 March, Surfer
   Work that makes you unhappy is what I mean by "a gnarly problem." The trouble is, the market pays for solutions to gnarly problems, not solutions to easy problems. As the Yorkshire lads say, "Where there's muck, there's brass." 2008, [Avram] Joel Spolsky, More Joel on Software: Further Thoughts on Diverse and Occasionally Related Matters that will Prove of Interest to Software Developers, Designers, and Managers, and to Those who, whether by Good Fortune or Ill Luck, Work with Them in Some Capacity, Berkeley, Calif.: Apress, page 152
4. (slang, US) Unpleasant, awful, ugly.
   We're not talking about a lame chick and a gnarly guy. We're talking about a couple of far-out dudes. 1981, Dan Jenkins, Baja Oklahoma, New York, N.Y.: Atheneum Books, ISBN 978-0-689-11173-0; reprinted New York, N.Y.: Pocket Books, 1982, ISBN 978-0-671-45163-9, page 269
   Funky Kong: 'Those Snowmads are a gnarly bunch! I hear they're enjoyin' your island. Go get 'em, DK!' 2014 February, Retro Studios, Donkey Kong Country: Tropical Freeze, Nintendo, level/area: Funky's Fly 'n' Buy
   1. (slang, US) Of music or a sound: harsh.
      [She] displays the same love of gnarly fuzztones and shout-it-out-loud choruses that began back in her daze with local all-girl rockers the Runaways. 12 October 1986, Los Angeles Times