"needlewoman, woman who sews or makes seams," 1640s, with -ess + seamster (also sempster) from Middle English semester "one who sews, one whose occupation is sewing," from Old English seamestre "sewer, tailor, person whose work is sewing," from seam (n.) + -ster.
The -ster ending is feminine, but in Old English seamestre also was applied to men, and the Middle English word was used of both sexes, though seamsters were "usually female" [Middle English Compendium]. Evidently by 17c. the fem. ending no longer was felt as such and a new one added (as in children, etc.), and seamster thence was applied to male sewers.
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, also obsolete rudesse, "lack of cultivation" (early 15c.).
For change of medial -l- to -r- in Old French, compare orme "elm" from Latin ulmus; chartre from cartula; chapitre from capitulum.