This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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v. To support or strengthen from beneath.

v. To strengthen, secure, or reinforce by passing a rope, cable, or chain around the underside of an object.

v. To give fundamental support; provide with a sound or secure basis; provide supportive evidence for.

v. To lend moral support to.

v. To secure below or underneath.

v. To bind below; to gird round the bottom.

To gird round the bottom; gird beneath.

v. make secure underneath

v. lend moral support to

"Many explanations have been suggested for the greater intolerance of more religiously observant people—that they are more likely to see the world in “we/they” or Manichaean terms, for example, or that moral absolutism—a clear distinction between right and wrong—is incompatible with the skeptical outlook that is said to undergird civic tolerance."

undermine   

undergirded    undergirding    undergirds    undergirt   

beef up    fortify    strengthen    support    back up   

undergirt