means the tendency to become annoyed, impatient, or angry easily, often over small things, especially when tired, hungry, or stressed.

from From Latin 'irritare,' to provoke or excite, which gave us 'irritate.' By the 18th century, scientists used 'irritability' as a technical term for the capacity of living tissue—muscles, nerves—to respond to a stimulus; physiologists prized it as a sign of life itself. The everyday sense, meaning quick-tempered touchiness, grew from that same root: a body or mood primed to react sharply to the slightest poke.