"A large soft hat covering all of the hair and typically having a decorative frill, worn indoors by women in the 18th and early 19th centuries", Mid 18th century: mob, variant of obsolete mab 'slut'. The word mob was first used in the sense 'prostitute' (mid to late 17th century), later denoting a negligee.[mob cap etymology, mob cap origin, 英语词源]