undergird

verb. to support from beneath.

verb. to provide foundational support, to uphold or strengthen from the beneath.

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Undergird can refer to architectural support or moral support. It generally refers to any support that is from the bottom up or foundational. If someone’s experiences with religion are rooted in their family, their faith is undergirded by their family. If a bridge is held up by suspension cords that connect to its underside, the suspension cords undergird the bridge.

And underneath, like moonlight fair,
    I saw a hundred fathoms deep,
The crystal columns light as air
    That undergird the Isle of Sleep.
—The Coming of the Princess, and Other Poems by Kate Seymour MacLean

Near Naxos, Brasidas, after vainly trying to make a friendly haven, bade his sailors undergird the ship with heavy cables, for the timbers seemed starting.
—A Victor of Salamis by William Stearns Davis

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Etymology, working backward:

• A combination of the English words under and gird.

  • Under

    • Old English under (same meaning)

      • Proto-Germanic under (see above)

        • Proto-Indo-European root ndher- (see above)

  • Gird

    • Old English gyrdan (to put a girdle on, to encircle, to bind)

      • Proto-Germanic gurdjan (same meaning)

        • Proto-Indo-European root gher- (to grasp, to enclose)

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Usage, according to the Google’s NGrams Viewer (may not be perfectly accurate):

For context, note that ‘the’ usually floats at ~4%.

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