The word "backstabbing" uses 12 letters: aabbbcgiknst.
backstabbing is a valid word in this word list. For a definition, see the external dictionary links below.
Try a search for backstabbing 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 backstabbing (in bold), or to aabbbcgiknst in any order:
s - backstabbings