This word is the superlative form of the word cutest. Usually used to describe a person that is unbelievably cute. There is no higher form of the word cute than this.
When one cannot figure out whether or not something is cute, but not that cute, go with cuteish. A word which can be described for many many things like cows. They are neither cute or attractive so they are just cuteish.
Person A: (posts picture of cow friends with description of, "Meet my cute(ish) animal friends)
Person B: (replies with:) "how dare you call them cute(ish)"
Person A: oh well, cows are not cute or not not cute, so therefore they are cuteish.