'attendress' was formed in English by adding the feminine suffix '-ess' (from Old French '-esse', ultimately from Late Latin '-issa') to the agent/verb 'attend' or the agent noun 'attendant'.

'attend' originates from Latin 'attendere' (ad- 'to/toward' + tendere 'to stretch/extend'); it passed into Old French as 'attendre' and into Middle English. The feminine formation with '-ess' produced 'attendress' in Modern English.

Initially and originally it meant 'a woman who attends or serves'; the basic meaning has remained the same, but the word is now archaic/rare and has largely been replaced by 'attendant' or more specific terms.