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

attoparsec
 n.

    About an inch. atto- is the standard SI prefix for multiplication by 10^
    -18. A parsec (parallax-second) is 3.26 light-years; an attoparsec is thus
    3.26 ? 10^-18 light years, or about 3.1 cm (thus, 1 attoparsec/
    microfortnight equals about 1 inch/sec). This unit is reported to be in
    use (though probably not very seriously) among hackers in the U.K. See 
    micro-.


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

attoparsec

    About 31 mm (one inch).  "atto-" is the
   standard SI prefix for multiplication by 10^-18.  A parsec
   (parallax-second) is 3.26 light-years; an attoparsec is thus
   3.26 * 10^-18 light years.  Thus, one attoparsec per
   microfortnight is about one inch per second.

   This unit is reported to be in use (though probably not very
   seriously) among hackers in the UK.

   [Jargon File]

   (1996-12-08)