Quick Definition
ἐπιορκέω v. I take an oath, swear falsely
Strong's Definition
from 1965; to commit perjury
KJV Usage: forswear self
UBS Dictionary of the Greek New Testament
verb | ὁρκίζω
33.464 · Communication · Swear, Put Under Oath, Vow
to forswear, to break an oath, to swear and fail to keep to swear that one will do something and then not fulfill the promise See also 33.465.
33.465 · Communication · Swear, Put Under Oath, Vow
to swear falsely, to perjure oneself to take an oath that something is true, when in reality one knows that it is false In the one occurrence in the NT of ἐπιορκέω[a] (33.464). In either case, it is often necessary to be quite specific about the meaning and to introduce sufficient information into the context so that the specific aspect of either breaking an oath or swearing falsely will be clear.
UBS Dictionary of the Greek New Testament © United Bible Societies, licensed CC BY-SA 4.0.
Thayer's Greek Lexicon
ἐπιορκέω, -@: fut. ἐπιορκήσω, cf. Kriiger § 40 s. v., and § 39, 12,4; [Veitch s.v.; B.53 (46)]; (émtopkos, q.v-) ; to swear falsely, forswear one’s self: Mt. v. 33. xiv. 28; 1 Esdr. i. 46; by Grk. writ. fr. [lom. down.)
Mounce Concise Greek Dictionary
ἐπιορκέω epiorkeō 1x
to forswear one’s self, to fail of observing one’s oath, Mt. 5:33*
Abbott-Smith Greek Lexicon
** ἐπιορκέω, -ῶ
(< ἐπίορκος), [in LXX: I Es 1:48, Wi 14:28 * ;]
to swear falsely, forswear oneself: Mt 5:33.†
Liddell-Scott-Jones — Greek-English Lexicon
ἐπιορκέω (epiorkeō) — to break an oath. ἐπιορκ-έω, also ἐφιορκέω [Refs 3rd c.BC+], etc.: future -ήσω [Refs 8th c.BC+]: aorist ἐπιώρκησα [Refs], infinitive -ορκῆσαι [Refs 5th c.BC+]: perfect ἐπιώρκηκα [Refs]:—swear falsely, forswear oneself, οὐδ᾽ ἐπιορκήσω πρὸς δαίμονος 8th c.BC: Ilias Homerus Epicus “Illiad” 19.188: also, c.accusative, of things sworn by, τὰς βασιληΐας ἱστίας ἐπιώρκηκε has sworn falsely by the royal hearth, [Refs 5th c.BC+]; [θεούς] [Refs 4th c.BC+]; opposed to εὐορκέω, Legal cited in [Refs]
II. simply, ={ὄμνυμι}, swear, [Refs]