Source : Webster's Revised Unabridged Dictionary (1913)

Vermin \Ver"min\, n. sing. & pl.; used chiefly as plural. [OE.
   vermine, F. vermine, from L. vermis a worm; cf. LL. vermen a
   worm, L. verminosus full of worms. See {Vermicular}, {Worm}.]
   1. An animal, in general. [Obs.]

            Wherein were all manner of fourfooted beasts of the
            earth, and vermin, and worms, and fowls. --Acts x.
                                                  12. (Geneva
                                                  Bible).

            This crocodile is a mischievous fourfooted beast, a
            dangerous vermin, used to both elements. --Holland.

   2. A noxious or mischievous animal; especially, noxious
      little animals or insects, collectively, as squirrels,
      rats, mice, flies, lice, bugs, etc. ``Cruel hounds or some
      foul vermin.'' --Chaucer.

            Great injuries these vermin, mice and rats, do in
            the field.                            --Mortimer.

            They disdain such vermin when the mighty boar of the
            forest . . . is before them.          --Burke.

   3. Hence, in contempt, noxious human beings.

            You are my prisoners, base vermin.    --Hudibras.

Source : WordNet®

vermin
     n 1: an irritating or obnoxious person [syn: {varmint}]
     2: any of various small animals or insects that are pests; e.g.
        cockroaches or rats