The Collaborative International Dictionary of English v.0.48:

Fum \Fum\, v. i.
   To play upon a fiddle. [Obs.]
   [1913 Webster]

         Follow me, and fum as you go.            --B. Jonson.
   [1913 Webster]

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

fum
 n.

    [XEROX PARC] At PARC, often the third of the standard metasyntactic
    variables (after foo and bar). Competes with baz, which is more
    common outside PARC.


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

fum

    At Xerox PARC, often the third standard
   metasyntactic variable after foo and bar.  baz is more
   common outside PARC.

   [Jargon File]

   (2003-09-24)