1. [n]  a disorderly crowd of people
2. [n]  an association of criminals; "police tried to break up the gang"; "a pack of thieves"
3. [n]  a loose affiliation of gangsters in charge of organized criminal activities
4. [adj]  characteristic of a mob; disorderly or lawless; "mob rule"; "fanned mounting tension into mobbish terrorizing"; "moblike mentality"
5. [v]  press tightly together or cram; "The crowd packed the auditorium"

MOB is a 3 letter word that starts with M.

1. \Mob\, n. [See {Mobcap}.]
   A mobcap. --Goldsmith.
   
   

2. \Mob\, v. t.
   To wrap up in, or cover with, a cowl. [R.]
   
   

3. \Mob\, n. [L. mobile vulgus, the movable common people. See
   {Mobile}, n.]
   1. The lower classes of a community; the populace, or the
      lowest part of it.
   
            A cluster of mob were making themselves merry with
            their betters.                        --Addison.
   
   2. Hence: A throng; a rabble; esp., an unlawful or riotous
      assembly; a disorderly crowd.
   
            The mob of gentlemen who wrote with ease. --Pope.
   
            Had every Athenian citizen been a Socrates, every
            Athenian assembly would still have been a mob.
                                                  --Madison.
   
            Confused by brainless mobs.           --Tennyson.
   
   {Mob law}, law administered by the mob; lynch law.
   
   {Swell mob}, well dressed thieves and swindlers, regarded
      collectively. [Slang] --Dickens.
   
   

4. \Mob\, v. t. [imp. & p. p. {Mobbed}; p. pr. & vb. n.
   {Mobbing}.]
   To crowd about, as a mob, and attack or annoy; as, to mob a
   house or a person.