Lexical Definition

‎ἐπάναγκες‎ epánankes (ep-an'-ang-kes) necessarily ^G:ADV
neuter of a presumed compound of ‎ἐπί‎ and ‎ἀνάγκη‎

necessary.

• (adverbially) on necessity, i.e. necessarily

Abbott-Smith

* ἐπανάγκης, -ες (< ἀνάγκη), only in neut.;
1. ἐπάναγκες (sc. ἐστί), it is compulsory, necessary.
2. As adv., of necessity: Ac 15:28.†

Abbott-Smith References

Liddell-Scott-Jones

Included with: ἐπάναγκες-ης,

used only in neuter:

__1 ἐπάναγκές [ἐστι] it is compulsory, necessary, with infinitive, [Refs 5th c.BC+]; μηδὲν ἐ. ἔστω let there be no compulsion,[Refs]
__2 as adverb, on compulsion, ἐ. κομῶντες wearing long hair by fixed custom, [Refs 5th c.BC+]; ἐ. λέγειν, ἐντίθεσθαι, [NT+4th c.BC+]

_{LSJ Scripture Refs: Acts.15.28}