The word "quintillionth" uses 13 letters: hiiillnnoqttu.
quintillionth is a valid word in this word list. For a definition, see the external dictionary links below.
Try a search for quintillionth in these online resources (some words may not be found):
Wiktionary - OneLook Dictionaries - Merriam-Webster - Google Search
Each search will normally open in a new window.
Words formed by adding one letter before or after quintillionth (in bold), or to hiiillnnoqttu in any order:
s - quintillionths