The headword is the aorist passive participle, feminine dative singular, of the verb te/mnw, I cut, I hew; see generally LSJ s.v. Hesychius has this same headword and (see n. 1 below) gloss, but in the nominative case -- probably quoted from Hesiod, Works and Days 420 (of timber). The dative case occurs in Christian liturgical poetry, referring to the splitting of the Red Sea.
[1] The gloss is the aorist passive participle, feminine dative singular (but see above), of the verb ko/ptw, I strike, I smite, I knock down; see generally LSJ s.v.