WordNet (r) 3.0 (2006):

ipse dixit
    n 1: an unsupported dogmatic assertion [syn: ipse dixit,
         ipsedixitism]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "ipse dixit":
   affirmance, affirmation, allegation, announcement, annunciation,
   appointment, assertion, asseveration, averment, avouchment, avowal,
   brevet, bull, conclusion, creed, declaration, decree, decree-law,
   decreement, decretal, decretum, dictum, diktat, edict, edictum,
   enunciation, fiat, law, manifesto, ordinance, ordonnance, position,
   position paper, positive declaration, predicate, predication,
   proclamation, profession, pronouncement, pronunciamento,
   proposition, protest, protestation, rescript, rule, ruling, say,
   say-so, saying, senatus consult, senatus consultum, stance, stand,
   statement, ukase, utterance, vouch, word