I read this definition when I was about 10 or 12. I forgot the word and have been looking ever since. I think I remember it starting with the letter "s".
Sounds like the adjective sesquipedalian. Merriam-Webster defines it as "having many syllables" (when used of words) or "given to or characterized by the use of long words" (when used of people).
There's also a noun sesquipedalianism.