The Collaborative International Dictionary of English v.0.48:

nano- \nan"o-\ (n[a^]n"[-o]-), pref.
   1. A prefix meaning one-billionth; as, a nanogram is
      one-thousandth of a microgram.
      [PJC]

   2. Very small; submicroscopic; -- used to designate sizes
      smaller than those that would be referred to as micro-;
      as, nanoscale manipulations.
      [PJC]

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

nano
 /nan'oh/, n.

    [CMU: from nanosecond] A brief period of time. ?Be with you in a nano?
    means you really will be free shortly, i.e., implies what mainstream people
    mean by ?in a jiffy? (whereas the hackish use of ?jiffy? is quite different
    ? see jiffy).