The Collaborative International Dictionary of English v.0.48:

lunatic fringe \lunatic fringe\ n.
   the members of a political or social group espousing extreme,
   fanatical, or seemingly irrational views.
   [PJC]

WordNet (r) 3.0 (2006):

lunatic fringe
    n 1: a political unit with extreme and fanatical views

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "lunatic fringe":
   Bolshevik, Jacobin, Wobbly, aficionado, anarchist,
   anarcho-syndicalist, buff, bug, crank, devotee, energumen,
   enthusiast, extreme left-winger, extremist, fan, fanatic, fanatico,
   freak, infatuate, left-wing extremist, mild radical, monomaniac,
   nihilist, nut, parlor Bolshevik, parlor pink, pink, pinko, radical,
   red, revolutionary, revolutionist, sans-culotte, subversive,
   syndicalist, ultra, ultraist, yippie, zealot



The Jargon File (version 4.4.7, 29 Dec 2003):

lunatic fringe
 n.

    [IBM] Customers who can be relied upon to accept release 1 versions of
    software. Compare heatseeker.


The Free On-line Dictionary of Computing (30 December 2018):

lunatic fringe

   [IBM] Customers who can be relied upon to accept release 1
   versions of software.

   [Jargon File]