Lexical Definition

‎ἄπιστος‎ ápistos (ap'-is-tos) unbelieving G:A
from ‎Α‎ (as a negative particle) and ‎πιστός‎

that believeth not, faithless, incredible thing, infidel, unbeliever(-ing).

Usage: 8 forms in 26 verses
Form Translated As     In Verse(s)
ἄπιστοσ unbeliever 1Cor.7.151Cor.14.24
unbelieving Matt.17.17Mark.9.19Luke.9.41John.20.271Cor.7.141Cor.7.14
ἄπιστον incredible Acts.26.8
unbelieving 1Cor.7.121Cor.7.13
ἄπιστοι unbelievers 1Cor.14.23
ἀπίστων unbelievers Luke.12.461Cor.6.6
unbelieving 1Cor.10.272Cor.4.4
ἀπίστου an unbeliever 2Cor.6.15
than an unbeliever 1Tim.5.8
ἀπίστοισ to unbelieving Rev.21.8
unbelieving 1Cor.14.221Cor.14.22Titus.1.15
with unbelievers 2Cor.6.14
απίστου unbelieving LXX.Prov.17.6
άπιστον an untrustworthy LXX.Isa.17.10LXX.Isa.17.10

Abbott-Smith

ἄ-πιστος, -ον (< ἀ- neg., πιστός), [in LXX: Pr 17:6 28:25, Is 17:10*;]
(a) of things, incredible: Ac 26:8;
(b) of persons, without faith or trust, unbelieving: Mt 17:17, Mk 9:19, Lk 9:41 12:46, Jo 20:27, Tit 1:15, Re 21:8; specif., of unbelievers as opp. to Christians: I Co 6:6 7:12-15 10:27 14:22-24, II Co 4:4 6:14, 15, I Ti 5:8 (cf. Lft., Notes, 265; Cremer, 491).†

Abbott-Smith References

Book Refs
Proverbs 17:6 · 28:25
Isaiah 17:10
Matthew 17:17
Mark 9:19
Luke 9:41 · 12:46
John 20:27
Acts 26:8
1 Corinthians 6:6 · 7:12 · 10:27 · 14:22
2 Corinthians 4:4 · 6:14 · 6:15
1 Timothy 5:8
Titus 1:15
Revelation 21:8

Liddell-Scott-Jones

ἄπιστος, ον,

__I passive, not to be trusted, and so:
__I.1 of persons and their acts, not trusty, faithless, ὑπερφίαλοι καὶ .[Refs 8th c.BC+]; δολοπλοκίαι[Refs 6th c.BC+]mistrusted, [ἄπιστος ἑταιρείας λιμήν 5th c.BC: Sophocles Tragicus “Ajax” 683">Refs 5th c.BC+]; θράσει . ἐπαιρόμενος by untrustworthy, groundless confidence, [Refs 5th c.BC+]; shifty, unreliable, [Refs 5th c.BC+]
__I.2 of reports and the like, incredible, uncertain in [Refs 7th c.BC+]; τὸ ἐλπίδων . undreamed of even in hope, [Refs 5th c.BC+]: comparative -ότερος, λόγος[Refs 4th c.BC+]: superlative, πίστις ἀπιστοτάτη[Refs 5th c.BC+]
__II active, mistrustful, incredulous, suspicious, θυμὸς δέ τοι αἰὲν .[Refs 8th c.BC+]less credulous, [Refs 5th c.BC+]; . πρὸς Φίλιππον distrustful towards him, [Refs 4th c.BC+]; . εἶ.. σαυτῷ you do not believe what you say yourself, [Refs 5th c.BC+]; τὸ ., ={ἀπιστία}, [Refs 5th c.BC+]
__II.b in NT, unbelieving, [NT]
__II.2 disobedient, disloyal, [Refs 5th c.BC+]; ἔχειν ἄπιστον.. ἀναρχίαν πόλει, i.e. ἀναρχίαν ἔχειν ἀπειθοῦσαν τῇ πόλει,[Refs 5th c.BC+]
__III adverb ἀπίστως:
__III.1 passive, beyond belief, . ἐπὶ τὸ μυθῶδες ἐκνενικηκότα[Refs 5th c.BC+]; οὐκ . not incredibly, [Refs 4th c.BC+]
__III.2 active, distrustfully, suspiciously, [Refs 5th c.BC+]
__III.2.b treacherously, [Refs 1st c.AD+]

LSJ Scripture Refs: 1Cor.6.6