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Undergird (verb) To strengthen, secure, or reinforce by passing a rope, cable, or chain around the underside of an object. 
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Undergird (verb) To give fundamental support; provide with a sound or secure basis; provide supportive evidence for. 
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Undergird (verb) To lend moral support to. 
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Undergird (verb) To secure below or underneath. 
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Underpin (verb) To support from below with props or masonry. “You should underpin the mine roof to prevent further collapse.” 
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Underpin (verb) To give support to; to corroborate. “Public confidence in politicians must underpin our democracy.” 
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Undergird (verb) secure or fasten from the underside, especially by a rope or chain passed underneath. 
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Undergird (verb) provide support or a firm basis for “that’s a philosophy that needs to undergird retailers’ business plans this year” 
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Underpin (verb) support (a building or other structure) from below by laying a solid foundation below ground level or by substituting stronger for weaker materials. 
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Underpin (verb) support, justify, or form the basis for “the theme of honour underpinning the two books” 
 
					