In a way that opposes or contradicts rational thinking; unreasonably or illogically.
From 'anti-' (against) + 'rationally' (in a rational manner), combining the prefix with the adverb form of 'rational' to describe irrational behavior.
People sometimes act antirationally because emotions hijack their brains—you might rationally know that snakes aren't dangerous, but fear makes you scream antirationally anyway!
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