The Collaborative International Dictionary of English v.0.48:

Unlawful \Un*law"ful\, a.
   Not lawful; contrary to law. -- Un*law"ful*ly, adv. --
   Un*law"ful*ness, n.
   [1913 Webster]

   Unlawful assembly. (Law) See under Assembly.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unlawfully
    adv 1: not conforming to the law; "they were unlawfully married"
           [ant: de jure, lawfully, legally]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

UNLAWFULLY, pleadings. This word is frequently used in indictments in the
description of the offence; it is necessary when the crime did not exist at
common law, and when a statute, in describing an offence which it creates,
uses the word, 1 Moody, Cr. Cas. 339; but it is unnecessary whenever the
crime existed at common law, and is manifestly illegal. 1 Chitty, Crim. Law,
*241; Hawk. B. 2, c. 95, s. 96; 2 Roll. Ab. 82; Bac. Abr. Indictment, G 1
Cro. C. C. 38, 43.