The Collaborative International Dictionary of English v.0.48:

Pentecosty \Pen`te*cos"ty\, n.; pl. Pentecosties. [Gr. ?, fr.
   ? the fiftieth, ? fifty.] (Gr. Antiq.)
   A troop of fifty soldiers in the Spartan army; -- called also
   pentecostys. --Jowett (Thucyd. ).
   [1913 Webster] Pentelic