undergird

The English verb gird means, among other things, "to encircle or bind with a flexible band." When undergird first entered English in the 16th century it meant "to make secure underneath," as by passing a rope or chain underneath something (such as a ship). That literal sense has long since fallen out of use, but in the 19th century undergird picked up the figurative "strengthen" or "support" sense that we still use. Gird and consequently undergird both derive from the Old English geard, meaning "enclosure" or "yard." Gird also gives us girder, a noun referring to a horizontal piece supporting a structure.