The Collaborative International Dictionary of English v.0.48:

Molossus \Mo*los"sus\, n. [L., fr. Gr. ?, prop., Molossian,
   belonging to the Molossians, a people in the eastern part of
   Epirus.] (Gr. & Lat. Pros.)
   A foot of three long syllables. [Written also molosse.]
   [1913 Webster]