From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

1: Ἀθέμιτος (Strong'S #111 — Adjective — athemitos — ath-em'-ee-tos )

a late form for athemistos (themis, "custom, right;" in classical Greek, "divine law"), "contrary to what is right," is rendered "an unlawful thing" (neuter) in  Acts 10:28; in  1—Peter 4:3 , "abominable."

 2—Peter 2:8Lawless.

King James Dictionary [2]

UNLAW'FUL, a. Not lawful contrary to law illegal not permitted by law.

Unlawful assembly, in law, the meeting of three or more persons to commit an unlawful act.

Webster's Dictionary [3]

(a.) Not lawful; contrary to law.