Lexical Definition

‎θνήσκω‎ thnḗskō (thnay'-sko) to die/be dead ^G:V
a strengthened form of a simpler primary (which is used for it only in certain tenses)

be dead, die.

• to die (literally or figuratively)

Abbott-Smith

θνήσκω, [in LXX chiefly for מות;]
to die; pf. (M, Pr., 114), to be dead: Mt 2:20, Mk 15:44, Lk 7:12 8:49, Jo 11:44 19:33, Ac 14:19 25:19; metaph., of spiritual death, I Ti 5:6 (cf. ἀπο-, συν-απο-θνήσκω).†

Abbott-Smith References

Liddell-Scott-Jones

θνῄσκω

(with ι [Refs]; θνείσκ- [Refs 5th c.BC+], Aeolic dialect θναίσκω Hdn.Gr.[Refs]θνᾴσκω [Refs 3rd c.BC+]: future θᾰνοῦμαι [Refs 5th c.BC+]; Epic dialect infinitive -έεσθαι[Refs 8th c.BC+]aorist 2 ἔθᾰνον, Epic dialect θάνον [Refs 8th c.BC+]; infinitive Epic dialect and Ionic dialect θανέειν, as always in [Refs 8th c.BC+]: perfect τέθνηκα [Refs 8th c.BC+]; subjunctive τεθνήκω [Refs 5th c.BC+]: pluperfect ἐτεθνήκειν [Refs 5th c.BC+]; 3rd.pers. plural -ήκεσαν[Refs 5th c.BC+]: short forms of perfect, 3rd.pers. dual τέθνᾰτον [Refs 5th c.BC+], 1st pers. plural τέθνᾰμεν [Refs 5th c.BC+], 3rd.pers. plural τεθνᾶσι [Refs 8th c.BC+]; 3rd.pers. plural pluperfect ἐτέθνᾰσαν [Refs 8th c.BC+]; infinitive τεθνάναι [ᾰ] [Refs 7th c.BC+] uncertain reading in [Refs 6th c.BC+]; Epic dialect τεθνάμεναι, -άμεν, [Refs 8th c.BC+]; Aeolic dialect τεθνάκην [Refs 7th c.BC+]; participle τεθνεώς [Refs 6th c.BC+]; genitive τεθνεῶτος, etc., [Refs 8th c.BC+]; poetry τεθνεότος Archel. cited in [Refs 3rd c.BC+]; Doric dialect τεθνᾰότα [Refs 5th c.BC+]; Epic dialect τεθνηώς (variant{-ειώς}) [Refs 8th c.BC+], -ηυῖα [Refs 8th c.BC+]; genitive τεθνηῶτος [Refs 8th c.BC+] is trisyllable [Refs 8th c.BC+]: disyllable forms are written in later Gr., nominative τεθνώς [Refs]; genitive singular τεθνῶτος [Refs 1st c.AD+]; dative singular τεθνῶτι [Refs]; feminine τεθνώσῃ (and genitive plural τεθνήτων) [Refs]; accusative plural feminine τεθνώσας [Refs 2nd c.AD+] arose future τεθνήξω [Refs 5th c.BC+] (censured as archaic by [Refs 2nd c.BC+]; participle τεθνηξόμενος [Refs 4th c.AD+]—The simple Verb is regularly used in early Prose in perfect and pluperfect; for present, future, and aorist the compound ἀποθνῄσκω is substituted: θνῄσκει variant in [Refs 5th c.BC+]: aorist participle θανών, subjunctive θάνῃ, [Refs 5th c.BC+]: aorist infinitive θανεῖν [Refs]:—in present and imperfect, die, as well of natural as of violent death; in aorist and perfect, to be dead (compare τί τοὺς θανὅντας οὐκ ἐᾷς τεθνηκένα; [Refs 8th c.BC+]; ζωὸς ἠὲ θανών alive or dead, [Refs 8th c.BC+]; τεθνάτω let him be put to death, [Refs]; ἄτιμος τεθ. Legal cited in [Refs 4th c.BC+]: frequently in participle, νέκυος πέρι τεθνηῶτος [Refs 8th c.BC+]; νεκρὸν.. τεθνηῶτα a dead corpse, [Refs 8th c.BC+]; οἱ τεθνηκότες, οἱ θανόντες, the dead, [Refs 5th c.BC+]; οἴχεται θανών (see. οἴχομαι); θανὼν φροῦδος (see. φροῦδος); θανόντι συνθανεῖν [Refs 5th c.BC+]; ὁ θανών, opposed to ὁ κτανών, [Refs 5th c.BC+]present with perfect sense, θνῄσκουσι γάρ, for τεθνήκασι, [Refs 5th c.BC+]

__2 used like a passive Verb, χερσὶν ὑπ᾽ Αἴαντος θανέειν to fall by his hand, [Refs 8th c.BC+]; πρός τινος[Refs 5th c.BC+]; τεθνάναι τῷ δέει, τ. τῷ φόβῳ, with accusative, to be in mortal fear of, [Refs 4th c.BC+]; θ. ἐπί τινι to die leaving one as heir, [Refs 2nd c.AD+]
__II metaphorically, of things, perish, θνᾴσκει σιγαθὲν καλὸν ἔργον [Refs 5th c.BC+]