what exactly is a mouth full?

Answer 1

I am going to assume you mean the expression: "That's a mouthful!" If so, it means that's a lot of words, or a lot of big words---so much so that all those words would fill your mouth all the way up!

Really, its just a humorous way of saying that something requires a lot of words to explain, or that even one word has a lot of letters in it.

A good example is the word "antidisestablishmentarianism." That is actually one real word, and if we try to say it , its a mouthful. Don't you think? But please, don't ask me to define it. That would be even more of a mouthful! LOL