The Collaborative International Dictionary of English v.0.48:

Natch \Natch\ (n[a^]ch), n. [OF. nache fesse, LL. natica, from
   L. natis the rump, buttocks. Cf. Aitchbone.]
   The rump of beef; esp., the lower and back part of the rump.
   [1913 Webster]

   Natch bone, the edgebone, or aitchbone, in beef.
      [1913 Webster]