The term "mostest" is not a proper word in English language and there are no real synonyms for it. It probably originated from a slang expression used to convey the idea of a complete or extreme degree of something. If we want to substitute "mostest" with a proper word, we could use "most" which means the greatest amount or degree of something. Other synonyms for "most" include "highest," "supreme," and "utmost." However, it's best to avoid using slang words in formal speech or writing and stick to proper English vocabulary.