Joan's as good as my lady in the dark

The story

From or alongside the use of Joan as a sexually available member of the lower classes found in related expressions such as He is our Ladies Chaplaine, but serves Ione and Joan's as good as this French lady. Ultimately a calque of Latin and Ancient Greek expressions such as πᾶσα γυνὲ τοῦ λύχνου ἀρθέντος ἡ αὐτή ἐστι (pâsa gunè toû lúkhnou arthéntos hē autḗ esti) and sublata enim lucerna quaevis mulier cuiusvis est similis (“all women are the same when the lamp is removed”), which Plutarch considered only accurate as an excuse to escape lecherous and adulterous men but which Erasmus considered a description of how all women would indulge themselves so long as witnesses and punishment could be avoided.