Why is the ninth month called September?
early 14c., "lower part of a woman's dress," from Old Norse skyrta "shirt, a kind of kirtle;" see shirt. Sense development from "shirt" to "skirt" is possibly related to the long shirts of peasant garb (cf. Low German cognate Schört, in some dialects "woman's gown"). Sense of "border, edge" (in outskirts, etc.) first recorded late 15c. Metonymic use for "women collectively" is from 1550s; slang sense of "young woman" is from 1906; skirt-chaser first attested 1942.
c.1600, "to border, form the edge of," from skirt (n.). Meaning "to pass along the edge" is from 1620s. Related: Skirted; skirting.