Why is the ninth month called September?
c.1200, lafdi, lavede, from Old English hlæfdige "mistress of a household, wife of a lord," literally "one who kneads bread," from hlaf "bread" (see loaf) + -dige "maid," related to dæge "maker of dough" (see dey (1); also compare lord). The medial -f- disappeared 14c. Not found outside English except where borrowed from it.
Sense of "woman of superior position in society" is c.1200; "woman whose manners and sensibilities befit her for high rank in society" is from 1861 (ladylike in this sense is from 1580s, and ladily from c.1400). Meaning "woman as an object of chivalrous love" is from early 14c. Used commonly as an address to any woman since 1890s. Applied in Old English to the Holy Virgin, hence many extended usages in plant names, place names, etc., from genitive singular hlæfdigan, which in Middle English merged with the nominative, so that lady- often represents (Our) Lady's; e.g. ladybug. Ladies' man first recorded 1784. Lady of pleasure recorded from 1640s.
["Key Concepts in the INCAS Multicomputer Project", J. Nehmer et al IEEE Trans Soft Eng SE-13(8):913-923 (Aug 1987)].
in the British Isles, a general title for any peeress below the rank of duchess and also for the wife of a baronet or of a knight. Before the Hanoverian succession, when the use of "princess" became settled practice, royal daughters were styled Lady Forename or the Lady Forename. "Lady" is ordinarily used as a less formal alternative to the full title of a countess, viscountess, or baroness; where the name is territorial, the "of " is dropped-thus the Vicountess of A. but Lady A. The daughters of dukes, marquesses, and earls also have, by courtesy, the title of lady prefixed to their forename and surname-e.g., Lady Jane Grey.