Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ămoebaeus, a, um, adj., = ἀμοιβαῖος. alternate (pure Lat. alternus); hence, amoebaeum carmen = ᾆσμα ἀμοιβαῖον, a responsive song, Fest.; Serv. ad Verg. E. 3, 28, 59, 66 al.
Hence in metre, pes amoebaeus: ex duabus longis et totidem brevibus et longā, Diom. p. 478 P. (e. g. īncrēdĭbĭlēs; opp. antamoebaeus, q. v.).