The Collaborative International Dictionary of English v.0.48:

beatnik \beat"nik\ n.
   a member of the Beat Generation; a nonconformist in dress and
   behavior.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

beatnik
    n 1: a member of the beat generation; a nonconformist in dress
         and behavior [syn: beatnik, beat]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "beatnik":
   Bohemian, deviant, dissenter, dropout, flower child, freak,
   heretic, hippie, maverick, misfit, nonconformist, nonjuror,
   original, sectarian, sectary, swinger, ugly duckling, unconformist,
   yippie