[uhn-der-mahyn or especially for 1, 2, 4, uhn-der-mahyn]
verb (used with object), undermined, undermining.
to injure or destroy by insidious activity or imperceptible stages, sometimes tending toward a sudden dramatic effect.
to attack by indirect, secret, or underhand means; attempt to subvert by stealth.
to make an excavation under; dig or tunnel beneath, as a military stronghold.
to weaken or cause to collapse by removing underlying support, as by digging away or eroding the foundation.

1300–50; Middle English underminen. See under-, mine2

underminer, noun
underminingly, adverb
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
undermine (ˌʌndəˈmaɪn)
1.  (of the sea, wind, etc) to wear away the bottom or base of (land, cliffs, etc)
2.  to weaken gradually or insidiously: their insults undermined her confidence
3.  to tunnel or dig beneath

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

c.1300, undermyne, from under + mine (v.). The fig. sense is attested from c.1430. Cf. Du. ondermijnen, Dan. underminere, Ger. unterminiren.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Undermining that kind of power can happen, but it normally takes wars, and it
  certainly takes generations.
As sea levels rise, persistent ocean waves reach farther inland, undermining
  property once well out of harm's way.
Termites may have been a severe problem, undermining the timbers that held up
  many structures.
In fact, they seem to be devoting considerable resources to undermining them.
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