1. to behave or conduct oneself in a way that lacks logic, reason, or sound judgment
He started to act irrationally after hearing the bad news.
Ele começou a agir irracionalmente depois de ouvir a má notícia.
2. to make decisions or take actions based on emotions rather than reason
She tends to act irrationally when she is angry.
Ela tende a agir irracionalmente quando está brava.