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Found one definition

1.                 From The Free On-line Dictionary of Computing (19 January 2023):
                   
   
   
                   
                     
   thud
      1. Yet another metasyntactic variable (see foo).  It is
      reported that at CMU from the mid-1970s the canonical series
      of these was "foo", "bar", "thud", "blat".
      2. Rare term for the hash character, "#" (ASCII 35).  See
      ASCII for other synonyms.
      [Jargon File]