Are you perhaps thinking of one of these words?
* Shyest: The superlative form of "shy," meaning the most timid or reserved.
* Sheist: A slang term for a con artist or swindler.
* Sliest: A less common alternative spelling of "slyest," meaning the most cunning or deceitful.
Please clarify what word you were trying to spell, and I can help you further.