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.7.15 • 1Cor.14.24
unbelieving Matt.17.17 • Mark.9.19 • Luke.9.41 • John.20.27 • 1Cor.7.14 • 1Cor.7.14
ἄπιστον incredible Acts.26.8
unbelieving 1Cor.7.12 • 1Cor.7.13
ἄπιστοι unbelievers 1Cor.14.23
ἀπίστων unbelievers Luke.12.46 • 1Cor.6.6
unbelieving 1Cor.10.27 • 2Cor.4.4
ἀπίστου an unbeliever 2Cor.6.15
than an unbeliever 1Tim.5.8
ἀπίστοισ to unbelieving Rev.21.8
unbelieving 1Cor.14.22 • 1Cor.14.22 • Titus.1.15
with unbelievers 2Cor.6.14
απίστου unbelieving LXX.Prov.17.6
άπιστον an untrustworthy LXX.Isa.17.10 • LXX.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 ἄ.[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 ἀ. 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 ἄ. 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 ἄ.[Refs 8th c.BC+]less credulous, [Refs 5th c.BC+]; ἄ. πρὸς Φίλιππον distrustful towards him, [Refs 4th c.BC+]; ἄ. εἶ.. σαυτῷ you do not believe what you say yourself, [Refs 5th c.BC+]; τὸ ἄ., ={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, ἀ. ἐπὶ τὸ μυθῶδες ἐκνενικηκότα[Refs 5th c.BC+]; οὐκ ἀ. 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