The Collaborative International Dictionary of English v.0.48:
umpteenth \ump"teenth\, a. Ordinal of umpteen, with corresponding signification; as, for the umpteenth time, I'm telling you to stop. [Colloq.] [PJC]WordNet (r) 3.0 (2006):
umpteenth adj 1: last in an indefinitely numerous series [syn: umpteenth, umteenth, umptieth]