A petticoat was a woman's light, loose undergarment hanging from the waist, worn under a skirt or dress.
Rosita Farisi wore a steel blue pinstripe petticoat when she helped the Tenth Doctor and Jackson Lake foil Cybus Cybermen during the 1851 Incident. (TV: The Next Doctor)
Missy, the feminine version of the Master and successor to the Saxon incarnation, wore a white petticoat. (TV: Empress of Mars)