The date or time when something stops being valid or usable, like when a passport or food expires.
From 'expire' (Latin 'expirare' meaning to breathe out or die) plus '-y' (noun suffix). The sense of 'breathing out' evolved to mean the end or conclusion of something.
The word literally means 'to breathe out'—like your soul leaving your body—which is beautifully dark when you think about your milk expiring! Many languages use this same 'ending/death' metaphor for when things stop being valid.
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