The Collaborative International Dictionary of English v.0.48:

Unrespectable \Unrespectable\
   See respectable.

WordNet (r) 3.0 (2006):

unrespectable
    adj 1: unworthy of respect [ant: respectable]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "unrespectable":
   derogatory, discreditable, disgraceful, dishonorable, disreputable,
   ignoble, ignominious, infamous, inglorious, notorious, seamy,
   shabby, shady, shameful, shoddy, sordid, unpraiseworthy,
   unsavory