The Collaborative International Dictionary of English v.0.48:

Willy nilly \Wil"ly nil"ly\
   Whether I (he, she, they) want to or not. See Will I, nill
   I, etc., under 3d Will.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Will \Will\, v. i.
   To be willing; to be inclined or disposed; to be pleased; to
   wish; to desire.
   [1913 Webster]

         And behold, there came a leper and worshiped him,
         saying, Lord if thou wilt, thou canst make me clean.
         And Jesus . . . touched him, saying, I will; be thou
         clean.                                   --Matt. viii.
                                                  2, 3.
   [1913 Webster]

   Note: This word has been confused with will, v. i., to
         choose, which, unlike this, is of the weak conjugation.
         [1913 Webster]

   Will I, nill I, or Will ye, hill ye, or Will he, nill
   he, whether I, you, or he will it or not; hence, without
      choice; compulsorily; -- commonly abbreviated to willy
      nilly. "If I must take service willy nilly." --J. H.
      Newman. "Land for all who would till it, and reading and
      writing will ye, nill ye." --Lowell.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "willy nilly":
   all over, all over hell, at intervals, brokenly, by catches,
   by fits, by jerks, by skips, by snatches, come what may,
   compulsorily, disconnectedly, discontinuously, every which way,
   fitfully, from necessity, haphazardly, harum-scarum, helplessly,
   helter-skelter, here and there, higgledy-piggledy, hugger-mugger,
   in a jumble, in a mess, in a muddle, in confusion, in disarray,
   in disorder, in spots, inescapably, inevitably, infrequently,
   intermittently, necessarily, need, needfully, needs, nolens volens,
   occasionally, of necessity, per saltum, perforce, randomly,
   requisitely, skimble-skamble, skippingly, unavoidably,
   without choice