From Latin 'indurare' (to make hard) — the same root gives us 'tree,' 'true,' and 'trust,' all meaning firmness.
To suffer through something painful or difficult without giving way; to continue to exist over time.
From Old French 'endurer' (to harden, to make firm, to last, to suffer patiently), from Latin 'indurare' meaning 'to make hard, to harden, to inure oneself,' composed of 'in-' (into, in) + 'durare' (to harden, to last, to persist), from the adjective 'durus' (hard, tough, unyielding, harsh). Latin 'durus' descends from PIE *deru- (firm, solid, steadfast; tree, oak), one of the most productively extended roots in the family. From this single root: Old English 'treow' (both tree and troth — the same word, because firmness and faithfulness were identical), giving
The PIE root *deru- (firm, solid, tree) connects 'endure' to 'tree,' 'true,' and 'trust' — all through the concept of firmness. What is 'true' is firm and reliable; what we 'trust' is solid as wood; what 'endures' has been hardened. The oak tree was the archetypal image of steadfastness across Indo-European