The Collaborative International Dictionary of English v.0.48:

Bacchius \Bac*chi"us\, n.; pl. Bacchii . [L. Bacchius pes, Gr.
   "o Bakchei^os (sc. poy`s foot).] (Pros.)
   A metrical foot composed of a short syllable and two long
   ones; according to some, two long and a short.
   [1913 Webster]