Attoparsec is an example of hacker or nerd humor.

The term attoparsec is composed of the SI prefix atto- and the SI unit parsec (parallax-second). It is, therefore, in a formal sense a perfectly legitimate SI unit with a proper abbreviation, apc. It is, however, rarely used.

Parsecs are used in astronomy to measure enormous interstellar distances; a parsec is approximately 3.26 light-years or 3.085×10¹⁶m. Combining it with the "atto" prefix yields attoparsec, a conveniently human-scaled unit of 3.085 centimeters (about 1-7/32 inches) that has no obvious practical use. It is used as a jocularity by programmers who are deliberately intending to obfuscate their code for humorous purposes. It is similar to the phrase "furlongs per fortnight", used by scientists and engineers as a joking reference meaning "any obscure unit."

Interestingly, 1 attoparsec/microfortnight is nearly 1 inch/second (the actual figure is 1.0043 inch per second).

A German counterpart is "km/Woche" (kilometers per week), said to be the nominal value for a snail's pace. As in "attoparsec," the joke consists of deliberately "blowing up" the fraction's numerator and denominator, instead of using the most simple form. One km/Woche = 1.65 mm/s. (This is reasonable for the European garden snail, Helix aspersa, whose speed is dependent on the surface and ranges from 1.05 mm/s on a paper towel to 2.8 mm/s on Formica. As of 2004 the record speed in the World Snail Racing Championships in Congham, U.K., set in 1995, is 13 inches in 2 minutes = 5.5 mm/s; the Costa Rican mountain snail Velifera can travel 18"/minute = 7.62 mm/s ).