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]

The Free On-line Dictionary of Computing (19 January 2023):

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)