The word "squish" refers to the action of compressing or squeezing something soft or wet, often causing it to change shape or break apart. It can also indicate the sound made when something soft is pressed or crushed. The term is commonly used to describe a sensation or sound associated with stepping on something like mud or a soft fruit.
The origin of "squish" is believed to be from a combination of the sounds associated with the action of squeezing, possibly influenced by the word "squash." The earliest known usage dates back to the early 20th century.