To weaken or damage something gradually, or to dig beneath the foundation of a structure to make it collapse.
From Middle English 'undermine,' combining 'under' (beneath) and 'mine' (to excavate). Originally referred to literal digging beneath fortifications in medieval warfare, then evolved to mean weakening anything systematically.
Medieval soldiers used undermining as a siege weapon—they'd dig tunnels under castle walls to make them collapse! Today when we say something 'undermines your confidence,' we're using the same metaphor of invisible damage from below.
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