From Latin 'indurare' (to make hard) — PIE *deru- (firm) also gave us 'tree,' 'true,' and 'trust.' Firmness unites them.
The ability to endure an unpleasant or difficult process or situation without giving way.
From Old French 'endurance,' from 'endurer' (to harden, to make last, to endure suffering), from Latin 'indūrāre' (to make hard, to harden, to render callous), from 'in-' (in, into, intensive prefix) + 'dūrāre' (to harden, to last, to persist), from 'dūrus' (hard, tough, unyielding), from PIE *deru- (to be firm, solid, steadfast — the root of hardness and trees). The same PIE root *deru- produced 'tree' (the firm-standing plant), 'true' (firm, reliable, Proto-Germanic *trewwaz), 'trust' (relying on what is firm), 'truce' (a firm agreement), 'endure,' 'durable,' 'duration,' 'during,' 'obdurate' (hardened against feeling), and 'indurate' (to make callous). In Gaelic the root appears in 'dair' (oak). The oak is the tree of trees, the hardest and most enduring — the PIE