The Collaborative International Dictionary of English v.0.48:

squish \squish\ (skw[i^]sh), n.
   A sound like that made by the feet of one walking in mud or
   slush; a gushing, swashing, or splashing sound; a squishing
   sound.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

squish \squish\ (skw[i^]sh), v. i. [Perh. imitative. Cf.
   Squelch.]
   To make the sound of squirting water, like that made by the
   feet of one walking in mud or slush; to make a kind of
   gushing, swashing, or splashing sound; to move with such a
   sound.
   [PJC]

WordNet (r) 3.0 (2006):

squish
    n 1: the noise of soft mud being walked on
    v 1: walk through mud or mire; "We had to splosh across the wet
         meadow" [syn: squelch, squish, splash, splosh,
         slosh, slop]
    2: put (a liquid) into a container or another place by means of
       a squirting action