One of the characteristics of living things, it is defined as the ability of an organism to respond to external stimuli like touch, heat, light, electricity, etc. Every living thing is irritable (some more so than others, especially during menopause - sorry mom, I had to say it!). Many simpler organisms like plants respond to stimuli by moving towards or away from the stimulus. Movement towards the stimulus is called a positive response, and movement away from it is called negative response.

So if someone calls you an irritable bitch, tell them that you're proud of it because it's what makes you alive.