early 14c., from Old French forteresse, forterece "strong place, fortification" (12c.), variant of fortelesse, from Medieval Latin fortalitia, from Latin fortis "strong" (see fort) + -itia, added to adjectives to form nouns of quality or condition. French -ess from Latin -itia also is in duress, largesse, riches. For change of medial -l- to -r- in Old French, compare orne "elm" from Latin ulmus; chartre from cartula; chapitre from capitulum.
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