The Petticoat , was A `small coat` made of soft cloth worn under the ARMOR to protect from chaffing and provide heat and absorb sweat. later it was worn under the doublet [ another name for Vest] Than it creeped from the male side to the female side of the closet,[ probbaly since people reused clothes many times], First it was worn as a chemise [ dress] slowly it sunk down the body and became the name for an underskirt.
mirjam