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

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "squish":
   assibilate, assibilation, atomize, bear, break into pieces,
   break to pieces, break up, buzz, crash, crowd, crunch, crush,
   cut to pieces, demolish, diffuse, disperse, disrupt, effervesce,
   effervescence, effervescing, fission, fizz, fizzle, fizzling,
   fragment, frication, frictional rustling, grind, hiss, hissing,
   hush, hushing, jam, lisp, make mincemeat of, mince, pulverize,
   push, rhonchus, scatter, shatter, shiver, shush, shushing,
   sibilance, sibilate, sibilation, siffle, sigmatism, siss, sissing,
   sizz, sizzle, sizzling, smash, smash up, sneeze, sneezing, sniff,
   sniffle, snore, snort, snuff, snuffle, spit, splinter, splutter,
   sputter, squab, squash, squeeze, squelch, sternutation, stertor,
   swish, wheeze, whish, whistle, whistling, white noise, whiz,
   whoosh, zip