Mob is yet another word which can mean promiscuous woman or slattern. I have mentioned on numerous occasions that the OED has hundreds, if not thousands, of words with this meaning (eg mystery, abbess, kitty, mouse and many others). Mob here is from a different root than the word mob meaning 'unruly crowd'. It is, however, linked to the word mob-cap, and it is believed to come from the old name Mab, short for Mabel. It also might have something to do with the obsolete verb mabble or the regional variant moble, which means 'to muffle or wrap up (the head)' (mob-caps cover all the hair; shower caps could be called mob-caps).
Mob meaning 'rabble' is short for the Latin expression mobile vulgus, the OED's definition of which is "mob, rabble; the common people, the populace". That definition has only just been changed this month. The 1989 edition definition was "The common mass of people; the lower orders; the uncultured or illiterate as a class; the populace, the masses".
The reason I was looking up mob in the dictionary was that I was editing a crossword clue that defined mob as 'a group of kangaroos'. That appears to be true, too, although kangaroos are not specifically mentioned in the OED, which just says "a flock, herd or drove of animals (in Australia and NZ)".