To undermine literally means to dig a hole underneath something, making it likely to collapse. But we more often use the word to describe sabotage or the act of weakening someone else's efforts.

Originally spelled with a ‘y’ instead of an ‘i’, undermine has Germanic roots and means "to weaken, hinder, or impair." Accidentally undermine the foundation of a house by digging a tunnel to China beneath it and you might be forgiven. Undermine your teacher’s authority by speaking out of turn and throwing spitballs and odds are you’ll get in trouble.

Definitions of undermine
  1. verb
    destroy property or hinder normal operations
    synonyms: counteract, countermine, sabotage, subvert, weaken
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    cause to run off the tracks
    type of:
    refuse to go along with; refuse to follow; be disobedient
  2. verb
    hollow out as if making a cave or opening
    synonyms: cave
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    excavate the earth beneath
    type of:
    core out, hollow, hollow out
    remove the interior of
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