Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
multĭ-mĕter, tri, m. [multus-metrum], many-metred, consisting of many feet or kinds of verse (post-class.): si poëmata illius metiaris, multimeter, argutus, artifex erat, Sid. Ep. 8, 11.