The New Hacker's Dictionary

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.

A well-known result in topology called the Brouwer Fixed-Point Theorem states that any continuous transformation of a 2-sphere 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.

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.

HAKMEM /hak'mem/ n.

MIT AI Memo 239 (February 1972). A legendary collection of neat mathematical and programming hacks contributed by many people at MIT and elsewhere. (The title of the memo really is "HAKMEM", which is a 6-letterism for 'hacks memo'.) Some of them are very useful techniques, powerful theorems, or interesting unsolved problems, but most fall into the category of mathematical and computer trivia. Here is a sampling of the entries (with authors), slightly paraphrased:

Item 41 (Gene Salamin): There are exactly 23,000 prime numbers less than 2^(18).

Item 46 (Rich Schroeppel): The most probable suit distribution in bridge hands is 4-4-3-2, as compared to 4-3-3-3, which is the most evenly distributed. This is because the world likes to have unequal numbers: a thermodynamic effect saying things will not be in the state of lowest energy, but in the state of lowest disordered energy.

Item 81 (Rich Schroeppel): Count the magic squares of order 5 (that is, all the 5-by-5 arrangements of the numbers from 1 to 25 such that all rows, columns, and diagonals add up to the same number). There are about 320 million, not counting those that differ only by rotation and reflection.