The word "backstabber" uses 11 letters: aabbbcekrst.
backstabber is a valid word in this word list. For a definition, see the external dictionary links below.
Try a search for backstabber in these online resources (some words may not be found):
Wiktionary - OneLook Dictionaries - Merriam-Webster - Google Search
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Words formed by adding one letter before or after backstabber (in bold), or to aabbbcekrst in any order:
s - backstabbers