- in a way that is contrary to reason or is not sensible; illogically: In firing a highly competent CEO without cause, the Board acted unlawfully, unreasonably, and irrationally.Stick to your simple calculation and logic, even when markets are behaving irrationally.
- in a way that is driven by impulse or instinct rather than reason; without having or being able to use the faculty of reason: When a reporter started poking around, the suspect began acting irrationally and took off like a wild animal.
