The Collaborative International Dictionary of English v.0.48:

Thud \Thud\ (th[u^]d), v. i. & t.
   To make, or strike so as to make, a dull sound, or thud.

         Hardly the softest thudding of velvety pads. --A. C.
                                                  Doyle.

         The waves break into spray, dash and rumble and thud
         below your feet.                         --H. F. Brown.
   [Webster 1913 Suppl.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thud \Thud\ (th[u^]d), n. [Cf. AS. [thorn][=o]den a whirlwind,
   violent wind, or E. thump.]
   A dull sound without resonance, like that produced by
   striking with, or striking against, some comparatively soft
   substance; also, the stroke or blow producing such sound; as,
   the thrud of a cannon ball striking the earth.
   [1913 Webster]

         At every new thud of the blast, a sob arose. --Jeffrey.
   [1913 Webster]

         At intervals there came some tremendous thud on the
         side of the steamer.                     --C. Mackay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thud
    n 1: a heavy dull sound (as made by impact of heavy objects)
         [syn: thump, thumping, clump, clunk, thud]
    v 1: make a dull sound; "the knocker thudded against the front
         door" [syn: thud, thump]
    2: strike with a dull sound; "Bullets were thudding against the
       wall"
    3: make a noise typical of an engine lacking lubricants [syn:
       crump, thud, scrunch]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "thud":
   beat, bump, chink, click, clink, clop, clump, clunk, crump,
   dull thud, flick, flump, hit, pad, pat, patter, pitapat,
   pitter-patter, plump, plunk, pop, pound, rap, smite, strike, tap,
   thump, tick, tinkle, tunk, wham, whomp



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

thud
 n.

    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 0100011). See ASCII for
    other synonyms.


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

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]