The headword is present active participle, masculine nominative singular, of the verb metasobe/w. Same entry in other lexica: see the references at Photius mu332 Theodoridis, where the editor notes that the source of the headword has been seen in Cyril of Alexandria, who [DW] exhibits far more instances of this verb than any other extant writer: see e.g. PG 76.197a (Against Nestorius 4.5) or PG 68.420b (De adoratione 6)
cf. generally sigma 761, sigma 762.