The property manifested by living organisms, and the elements and tissues of which they are constituted, of responding to the action of stimulants; irritability; as, nervous excitability
If you describe someone as excitable, you mean that they behave in a rather nervous way and become excited very easily. Mary sat beside Elaine, who today seemed excitable + excitability ex·cit·abil·ity She has always been inclined to excitability. becoming excited too easily