Sometime in the 15th century, English speakers began to use stalwart in place of the older form stalworth. Although stalworth is now archaic, it laid the groundwork for today's meaning of stalwart. In the 12th century, stalworth began to be used to describe strongly built people or animals (a meaning stalwart took on about two centuries later).

Etymology: [OE. stalworth, AS. staelwyr serviceable, probably originally, good at stealing, or worth stealing or taking, and afterwards extended to other causes of estimation. See Steal, v. t., Worth, a.]