early 15c., pety coote, literally "a small coat," from petty + coat (n.). Originally a padded coat worn by men under armor, applied mid-15c. to a garment worn by women and young children. By 1590s, the typical feminine garment, hence a symbol of female sex or character.
Men declare that the petticoatless female has unsexed herself and has left her modesty behind. ["Godey's Magazine," April 1896]