A well-known result in topology called the Brouwer Fixed-Point Theorem states that any continuous transformation of a surface into itself has at least one fixed 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."
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.