The Collaborative International Dictionary of English v.0.48:

Hairy \Hair"y\ (-[y^]), a.
   1. Bearing or covered with hair; made of or resembling hair;
      rough with hair; hirsute.
      [1913 Webster]

            His mantle hairy, and his bonnet sedge. --Milton.
      [1913 Webster]

   2. Very complicated, difficult, or involved; as, a hairy
      problem; a hairy equation. [Colloq.]
      [PJC]

   3. Dangerous or frightening; as, a hairy encounter with a
      mugger.
      [PJC]

WordNet (r) 3.0 (2006):

hairy
    adj 1: having or covered with hair; "Jacob was a hairy man"; "a
           hairy caterpillar" [syn: hairy, haired, hirsute]
           [ant: hairless]
    2: hazardous and frightening; "hairy moments in the mountains"

Moby Thesaurus II by Grady Ward, 1.0:

145 Moby Thesaurus words for "hairy":
   Herculean, abstruse, arduous, asperous, barbellate, barfy, bearded,
   bewhiskered, bristled, bristling, bristly, brutal, bum, bushy,
   capillary, chancy, cheesy, cirrose, cirrous, comate, complex,
   complicated, confused, confusing, craggy, crappy, creepy, critical,
   crummy, dangerous, delicate, demanding, difficile, difficult,
   dirty, downy, exacting, fibered, fibroid, fibrous, flagelliform,
   fleecy, flocculent, flossy, fluffy, formidable, frightening,
   fringy, funicular, furry, fuzzy, glochidiate, godawful, goshawful,
   gossamery, grim, hairlike, hard, hard-earned, hard-fought, harsh,
   hazardous, hirsute, hispid, icky, intricate, jagged, jawbreaking,
   jeopardous, knotted, knotty, laborious, lanate, ligamental, matted,
   mean, nappy, nerve-racking, no picnic, not easy, operose, perilous,
   pilose, precarious, problematic, pubescent, punk, putrid, rigorous,
   risky, ropy, rough, rugged, scabrous, scary, scraggy,
   set with thorns, setaceous, setose, setous, severe, shagged,
   shaggy, shitty, silky, spiny, steep, stinking, stinky, strenuous,
   strigal, strigate, strigose, stringy, stubbled, stubbly, studded,
   taeniate, taeniform, tangled, thorny, threadlike, thready,
   ticklish, toilsome, tough, treacherous, trichoid, tricky, tufted,
   uncertain, uneven, unhealthy, unshaven, unshorn, unsmooth, unsound,
   uphill, vomity, whiskered, wicked, wiry, woolly, worrying,
   yecchy



The Jargon File (version 4.4.7, 29 Dec 2003):

hairy
 adj.

    1. Annoyingly complicated. ?DWIM is incredibly hairy.?

    2. Incomprehensible. ?DWIM is incredibly hairy.?

    3. Of people, high-powered, authoritative, rare, expert, and/or
    incomprehensible. Hard to explain except in context: ?He knows this hairy
    lawyer who says there's nothing to worry about.? See also hirsute.

    There is a theorem in simplicial homology theory which states that any
    continuous tangent field on a 2-sphere is null at least in a point.
    Mathematically literate hackers tend to associate the term ?hairy? with the
    informal version of this theorem; ?You can't comb a hairy ball smooth.?
    (Previous versions of this entry associating the above informal statement
    with the Brouwer fixed-point theorem were incorrect.)

    The adjective ?long-haired? is well-attested to have been in slang use
    among scientists and engineers during the early 1950s; it was equivalent to
    modern hairy senses 1 and 2, and was very likely ancestral to the hackish
    use. In fact the noun ?long-hair? was at the time used to describe a person
    satisfying sense 3. Both senses probably passed out of use when long hair
    was adopted as a signature trait by the 1960s counterculture, leaving
    hackish hairy as a sort of stunted mutant relic.

    In British mainstream use, ?hairy? means ?dangerous?, and consequently, in
    British programming terms, ?hairy? may be used to denote complicated and/or
    incomprehensible code, but only if that complexity or incomprehesiveness is
    also considered dangerous.


The Free On-line Dictionary of Computing (30 December 2018):

hairy

   1. Annoyingly complicated.  "DWIM is incredibly hairy."

   2. Incomprehensible.  "DWIM is incredibly hairy."

   3. Of people, high-powered, authoritative, rare, expert,
   and/or incomprehensible.  Hard to explain except in context:
   "He knows this hairy lawyer who says there's nothing to worry
   about."  See also hirsute.

   The adjective "long-haired" is well-attested to have been in
   slang use among scientists and engineers during the early
   1950s; it was equivalent to modern "hairy" and was very likely
   ancestral to the hackish use.  In fact the noun "long-hair"
   was at the time used to describe a hairy person.  Both senses
   probably passed out of use when long hair was adopted as a
   signature trait by the 1960s counterculture, leaving hackish
   "hairy" as a sort of stunted mutant relic.

   4.  hairy ball.

   [Jargon File]

   (2001-03-29)