The Collaborative International Dictionary of English v.0.48:

Choriambus \Cho`ri*am"bus\, n.; pl. L. Choriambi, E.
   Choriambuses. [L. choriambus, Gr. ?; ? a choreus + ?
   iambus.] (Anc. Pros.)
   A foot consisting of four syllables, of which the first and
   last are long, and the other short (- [crescent] [crescent]
   -); that is, a choreus, or trochee, and an iambus united.
   [1913 Webster]