G0226
ἀληθεύω
be truthful
ἀληθεύω (ἀληθής), [in LXX: Gen.20:16 (יָכַח) Gen.42:16 (אֶמֶת), Pro.21:3 (מִשְׁפָּט), Isa.44:26 (שָׁלַם), Sir.34:4 * ;] to speak the truth (R, mg., deal truly; Field, Notes, 192): Gal.4:16, Eph.4:15.† (AS)
Full Thayer's lexicon entry
ἀληθεύω — alētheúō
ἀληθεύω; in secular writings ((Aeschylus), Xenophon, Plato, Aristotle, others) to speak the truth;
a. to teach the truth: τίνι Gal 4:16.
b. to profess the truth (true doctrine): Eph 4:15. (R. V. marginal reading in both passages, to deal truly.)
2 occurrences
Galatians (1), Ephesians (1)