The headword as transmitted is dative singular of a putative neuter noun me/satmon, but no such noun is otherwise attested. In fact it occurs in the Septuagint (1 Samuel 17:7), with the same meaning, as me/saklon; cf. LSJ s.v. The word also appears in Pausanias the Atticist (mu13) and Hesychius (mu96) as me/sakmon, and Hesychius mentions as variants mesa/ktwn and mesa/krwn.
The dative being glossed here is possibly mesa/kmw| in Nonnus, Dionysiaca 24.254. Although LSJ gives me/saklon as the primary form, a)/kmwn at least provides an etymology for the word ("middle 'anvil'?"); and Eustathius, Commentary on Odyssey 8.107, explicitly posits mesa/kmwn.