The word "backcloths" uses 10 letters: abcchklost.
backcloths is a valid word in this word list. For a definition, see the external dictionary links below.
Try a search for backcloths in these online resources (some words may not be found):
Wiktionary - OneLook Dictionaries - Merriam-Webster - Google Search
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Shorter words found within backcloths