Lexical Definition

‎ἀληθεύω‎ alētheúō (al-ayth-yoo'-o) be truthful G:V
from ‎ἀληθής‎

speak (tell) the truth.

Usage: 6 forms in 6 verses
Form Translated As     In Verse(s)
ἀληθεύων speaking truth Gal.4.16
ἀληθεύοντεσ Speaking the truth Eph.4.15
αληθεύων verifying LXX.Isa.44.26
αληθεύετε you be truthful LXX.Gen.42.16
αληθεύειν to be truthful LXX.Prov.21.3
αλήθευσον you be truthful LXX.Gen.20.16

Abbott-Smith

ἀληθεύω (< ἀληθής), [in LXX: Ge 20:16 (יָכַח) Ge 42:16 (אֶמֶת), Pr 21:3 (מִשְׁפָּט), Is 44:26 (שָׁלַם), Si 34:4*;]
to speak the truth (R, mg., deal truly; Field, Notes, 192): Ga 4:16, Eph 4:15.†

Abbott-Smith References

Book Refs
Genesis 20:16 · 42:16
Proverbs 21:3
Isaiah 44:26
Galatians 4:16
Ephesians 4:15

Liddell-Scott-Jones

ἀληθεύω-εύω,

future -εύσω[Refs 5th c.BC+]:—speak truth, [Refs 5th c.BC+]adjective, . πάντα speak truth in all things, [Refs 5th c.BC+]; τὰς δέκα ἡμέρας ἠλήθευσε he rightly foretold.. , [Refs]; . τοὺς ἐπαίνους prove their praises true, [Refs 2nd c.AD+]; τοὔνομα `make good', [Refs 4th c.AD+]

__2 of things, to be, prove true, σημεῖα[Refs 5th c.BC+]:—passive, to be fulfilled, of conditions, ἐπὶ τούτοις -ομένοις[Refs 5th c.BC+]:—active of reasoners, arrive at truth, [Refs]:—passive, λόγος -εύεται is in accordance with truth, Top. [Refs]; ἀληθεύωεύεσθαι κατά τινος to be truly predicated of.., [Refs]future middle in same sense, [Refs]