The headword is future active infinitive of meqi/hmi. Same or similar entry in other lexica (see the references at Photius mu190 Theodoridis), and cf. a scholion on Homer, Iliad 6.330 (where the participle meqie/nta occurs). The headword itself must be quoted from somewhere else; perhaps Euripides (Medea 751) or Plato (Laws 951D), though there are numerous later possibilities.
See also mu 417, mu 422, mu 423, mu 427.