Emily Dickinson Lexicon

MET'TLE, n. [met'l; usually supposed to be corrupted from inmetal. But it may be from W. mezwl or methwl, mind, connected with mezu, to be able, and coinciding with the root of the Eng. moody; D. moed, courage, heart, spirit; G. muth, mind, courage; mettle; Sax. mod; Sw. mod; Dan. mod or mood; Goth. mod, angry. The Sax. modig, L. animus, animosus, furnish an analogy in point. The radical sense of mind, is to advance, to push forward, whence the sense of briskness, ardor.]

Spirit; constitutional ardor; that temperament which is susceptible of high excitement. It is not synonymous with courage, though it may be accompanied with it, and is sometimes used for it. The winged courser, like a generous horse, / Shows most true mettle when you check his course. Pope.