What does "gizzard" mean?

Gizzard: (Zoöl.) (a) A thick muscular stomach found in many invertebrate animals. (b) A stomach armed with chitinous or shelly plates or teeth, as in certain insects and mollusks. Gizzard shad (Zoöl.), an American herring (Dorosoma cepedianum) resembling the shad, but of little value. -- To fret the gizzard, to harass; to vex one's self; to worry. [Low] Hudibras. -- To stick in one's gizzard, to be difficult of digestion; to be offensive. [Low]