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. <topology> hairy ball.