Q:

What does "smoosh" mean?

I read this sentence in one of my son's picture books:

"The tire was squashed, the tire was smooshed."

(It's talking about a bike). 
The word "smoosh" is strange to me, so I consulted some dictionaries and it surprised me that I couldn't even find an entry for "smoosh".
What does it mean? How come it can't be found in dictionaries but can be found in a kid's book???

A:

From Oxford:

Smoosh (v): squash, crush, or flatten.
‘use a sharp knife so as not to smoosh the broccoli’

