Douglas Harper's Etymology Dictionary
mob-cap
a type of woman's indoor cap, 1795 (as simply mob, 1748), from cap (n.) + obsolete mob (n.) "negligent attire" (1660s), earlier "a strumpet" (earlier form mab, 1550s), related to obsolete verb mob "to tousle the hair, to dress untidily" (1660s), and perhaps ultimately from mop, but influenced by Mab as a female name. Dutch has a similar compound, mopmuts, but the relationship between it and the English word is uncertain.
Usage examples of "mob-cap".
The mob-capped heads of two of the parlormaids emerged through the banisters on the second-floor balcony.
A spinster is a dried-up old maid in a mob-cap, sitting in the chimney corner winding flax on a distaff.