means The outer parts of a town or city, lying away from the center where buildings thin out into edges and open land.

from A plain compound of "out" plus "skirts" — the plural of "skirt," itself from Old Norse "skyrta," a shirt or garment, a cousin of the English word "shirt." The idea is geographic clothing: just as a skirt's hem hangs at the body's edge, the "skirts" of a place are its outer border. "Outskirts" simply pushes that hem further out, naming the fringe where a settlement trails off.