umpteenth

from WordNet (r) 3.0 (2006)

umpteenth
    adj 1: last in an indefinitely numerous series [syn:
           {umpteenth}, {umteenth}, {umptieth}]

from 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]