squirrel scout cookie
It is self explanatory slang, referencing the exchange of one word (squirrel) with another (girl) in the name of a popular scout club that, in real life, sell cookies. It is merely a play on words and how they can sound so funny together.
As we walked up to the storefront Wallace said said "Hey look! The Squrrel Scouts are here selling their cookies" to wit he exclaimed "Squirrel Scout Cookies"!