Putting effort or force into something, usually physical or mental work needed to accomplish a goal.
From Latin 'exserere' meaning 'to thrust out' (ex- 'out' + serere 'to join'). It evolved to mean applying force or effort in English by the 1600s.
The root 'serere' connects to 'series'—when you exert yourself, you're literally 'thrusting out' a series of efforts, which is why exhaustion feels like you've pushed yourself beyond your connections.
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