Source : Webster's Revised Unabridged Dictionary (1913)

Irritable \Ir"ri*ta*ble\, a. [L. irritabilis: cf. F. irritable.
   See {Irritate}.]
   1. Capable of being irriated.

   2. Very susceptible of anger or passion; easily inflamed or
      exasperated; as, an irritable temper.

            Vicious, old, and irritable.          --Tennyson.

   3. (Physiol.) Endowed with irritability; susceptible of
      irritation; capable of being excited to action by the
      application of certain stimuli.

   4. (Med.) Susceptible of irritation; unduly sensitive to
      irritants or stimuli. See {Irritation}, n., 3.

   Syn: Excitable; irascible; touchy; fretful; peevish.

Source : WordNet®

irritable
     adj 1: easily irritated or annoyed; "an incorrigibly fractious
            young man"; "not the least nettlesome of his
            countrymen" [syn: {cranky}, {fractious}, {nettlesome},
             {peevish}, {peckish}, {pettish}, {petulant}, {testy},
             {tetchy}, {techy}]
     2: abnormally sensitive to a stimulus
     3: capable of responding to stimuli [syn: {excitable}]