As sound as a roach

from The Collaborative International Dictionary of English v.0.48

Roach \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. (Zool.)
      (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.
          [1913 Webster]

   2. (Naut.) A convex curve or arch cut in the edge of a sail
      to prevent chafing, or to secure a better fit.
      [1913 Webster]

   {As sound as a roach} [roach perhaps being a corruption of a
      F. roche a rock], perfectly sound.
      [1913 Webster]