The Collaborative International Dictionary of English v.0.48:

Amphimacer \Am*phim"a*cer\, n. [L. amphimacrus, Gr. ?; 'amfi` on
   both sides + ? long.] (Anc. Pros.)
   A foot of three syllables, the middle one short and the
   others long, as in c[=a]st[i^]t[=a]s. --Andrews.
   [1913 Webster]