the.com/underminer

means Someone who quietly weakens your position, confidence, or plans through small sabotaging acts rather than open opposition.

from From 'undermine' — literally to dig a tunnel or mine beneath a wall, castle, or fortification so it collapses from below. Military engineers did this literally for centuries: dig under the enemy's walls, remove the support, watch them crumble without a direct assault. By the 1500s the word had already tunneled its way into figurative use — weakening someone's authority, health, or resolve by stealth rather than force. The 'un-' here isn't negation; it's 'under,' doing exactly what it says.