The Collaborative International Dictionary of English v.0.48:

Unprincipled \Un*prin"ci*pled\, a. [Pref. un- not + principled.]
   Being without principles; especially, being without right
   moral principles; also, characterized by absence of
   principle. -- Un*prin"ci*pled*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unprincipled
    adj 1: lacking principles or moral scruples; "freedom from
           coarse unprincipled calumny"- A.E.Stevenson [ant:
           principled]
    2: having little or no integrity

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "unprincipled":
   amoral, conscienceless, corrupt, corrupted, criminal, crooked,
   dark, devious, dishonest, dishonorable, dissolute, doubtful,
   dubious, evasive, felonious, fishy, fraudulent, ill-got,
   ill-gotten, immoral, indirect, insidious, licentious, mercenary,
   not kosher, profligate, questionable, reprobate, rotten, shady,
   shameless, shifty, sinister, slippery, stick-at-nothing,
   suspicious, tricky, unconscienced, unconscientious, unconscionable,
   underhand, underhanded, unethical, unsavory, unscrupulous,
   unstraightforward, venal, without remorse, without shame