The Collaborative International Dictionary of English v.0.48:

Moo \Moo\ (m[=o]), a., adv., & n.
   See Mo. [Obs.] --Chaucer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Moo \Moo\ (m[=oo]), v. i. [imp. & p. p. Mooed; p. pr. & vb. n.
   Mooing.] [Of imitative origin.]
   To make the noise of a cow; to low; -- a child's word.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Moo \Moo\, n.
   The lowing of a cow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

moo
    n 1: the sound made by a cow or bull
    v 1: make a low noise, characteristic of bovines [syn: moo,
         low]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "moo":
   bark, bawl, bay, bell, bellow, blare, blat, blate, bleat, bray,
   call, caterwaul, cry, give tongue, give voice, howl, low, meow,
   mew, mewl, miaow, neigh, nicker, pule, roar, screak, scream,
   screech, squall, squeak, squeal, troat, ululate, wail, whicker,
   whine, whinny, yap, yawl, yawp, yelp, yip, yowl



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

MOO
       MUD Object Orientated (Internet, OOP, MUD)


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

MUD Object Oriented
MOO

    (MOO) One of the many MUD spin-offs (e.g. MUSH,
   MUSE, and MUX) created to diversify the realm of
   interactive text-based gaming.  A MOO is similar to a MUSH in
   that the users themselves can create objects, rooms, and code
   to add to the environment.

   The most frequently used server software for running a MOO
   is LambdaMOO but alternatives include WinMOO
   (http://www-personal.umich.edu/~cunkel/WinMOO/) and
   MacGoesMOO
   (http://neon.ci.lexington.ma.us/SpamCentral/scaron/mgm.html).

   (1999-05-25)