Noah Webster's New International Dictionary of the English Language

1. (a.) ill-tempered; impatient; easily inflamed or exasperated; as, an irritable temper.

2. (a.) Very susceptible of anger or passion

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

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