Vine's Expository Dictionary of NT Words 
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."
King James Dictionary 
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 
(a.) Not lawful; contrary to law.