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

automagically
 /aw?toh?maj'i?klee/, adv.

    Automatically, but in a way that, for some reason (typically because it is
    too complicated, or too ugly, or perhaps even too trivial), the speaker
    doesn't feel like explaining to you. See magic. ?The C-INTERCAL compiler
    generates C, then automagically invokes cc(1) to produce an executable.?

    This term is quite old, going back at least to the mid-70s in jargon and
    probably much earlier. The word ?automagic? occurred in advertising (for a
    shirt-ironing gadget) as far back as the late 1940s.


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

automagically

    /aw-toh-maj'i-klee/ or /aw-toh-maj'i-k*l-ee/
   Automatically, but in a way that, for some reason (typically
   because it is too complicated, or too ugly, or perhaps even
   too trivial), the speaker doesn't feel like explaining to you.

   E.g. "The C-INTERCAL compiler generates C, then automagically
   invokes cc to produce an executable."

   See magic.

   [Jargon File]

   (2001-05-18)