Source : Webster's Revised Unabridged Dictionary (1913)

Amphimacer \Am*phim"a*cer\, n. [L. amphimacru?, Gr. ?; ? on both
   sides + ? long.] (Anc. Pros.)
   A foot of three syllables, the middle one short and the
   others long, as in c[=a]st?t[=a]s. --Andrews.