means Easily annoyed, prone to sudden anger or impatience over small things.

from From Latin 'irritabilis,' from the verb 'irritare,' meaning 'to provoke, excite, or enrage.' English borrowed it in the 1600s, first as a medical and physiological term — tissues and nerves were called 'irritable' if they twitched or reacted to a stimulus. Only later did the word leak out of anatomy textbooks and into the realm of moods, where it has been twitching reactively ever since.