Roach, n. [OE. rroche; cf. AS. reohha, D. rog, roch, G. roche, LG. ruche, Dan. rokke ray, Sw. rocka, and E. ray a fish.] 1. (Zoöl.) (a) A European fresh-water fish of the Carp family (Leuciscus rutilus). It is silver-white, with a greenish back. (b) An American chub (Semotilus bullaris); the fallfish. (c) The redfin, or shiner.
2. (Naut.) A convex curve or arch cut in the edge of a sail to prevent chafing, or to secure a better fit.
As sound as a roach [roach perhaps being a corruption of a F. roche a rock], perfectly sound.