Emily Dickinson Lexicon

ARC, n. [L. arcus, a bow, vault, or arch; arcuo, to bend; Gr. αρχη, beginning, origin; αρχω, to begin, to be the author or chief; Fr. arc, arche; Sp. arco, a bow and arch; Port. id.; It. id.; Arm. goarec. The Greek word has a different application, but is probably from the same root as arcus, from the sense of springing or stretching, shooting up, rising, which gives the sense of a vault, or bow, as well as of chief or head. Hebrew, ארג, to weave; Syriac, ܐܪܓ to desire, or long for; Ar. أَرِجَ aricha, ariga, to emit odor, to diffuse fragrance; and Heb. ערג, to desire, or long for, to ascend; Eth. ዐረገ to ascend, to mount; Ar. id. The radical sense of all these roots is, to stretch, strain, reach; Gr. ορεγω; L. fragro; and the sense of arch is from stretching upward, ascending. From arc or arch comes the sense of bending, deviating, and cunning.]

In geometry, any part of the circumference of a circle, or curved line, lying from one point to another; a segment, or part of a circle, not more than a semicircle. – Encyc. Johnson.