[1] Likewise in other lexica. The primary headword is aorist middle, third person singular, of a)mpe/xw (cf. alpha 1652). It must be quoted from somewhere; perhaps (as Latte on Hesychius claims) from Euripides, Medea 1159 (web address 1); alternatively, as suggested by Theodoridis on Photius s.v., from Aristophanes, Thesmophoriazusae 165 (though some favour the reading h)mpi/sxeto there).
[2] Imperfect middle of the related verb a)mpi/sxw, third person singular, evidently quoted from somewhere.
[3] Imperfect middle of a)mpe/xw, third person singular, with double augment, perhaps from Plato, Phaedo 87B (web address 2).