Cyclopedia of Biblical, Theological and Ecclesiastical Literature
(ὑποζώννυμι , lit. to gird under the breast, 2 Macc. 3, 19; comp. 2Elian, V. H. 10:22), a naval term employed (Acts 27:17) to designate the act of passing cables around the middle of a ship in order to strengthen it (so Polybius, 27:3, 3; Appian, Bell. 104. 5, 91; Plato, Rep. p. 616). (See SHIP).
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McClintock, John. Strong, James. Entry for 'Undergird'. Cyclopedia of Biblical, Theological and Ecclesiastical Literature. https://www.studylight.org/encyclopedias/eng/tce/u/undergird.html. Harper & Brothers. New York. 1870.