The Collaborative International Dictionary of English v.0.48:

Wittol \Wit"tol\, n. [Said to be for white tail, and so called
   in allusion to its white tail; but cf. witwal.]
   [1913 Webster]
   1. (Zool.) The wheatear. [Prov. Eng.]
      [1913 Webster]

   2. A man who knows his wife's infidelity and submits to it; a
      tame cuckold; -- so called because the cuckoo lays its
      eggs in the wittol's nest. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wittol
    n 1: an archaic term for a cuckold who knows about his wife's
         infidelity but tolerates it