In a way that breaks the law; doing something illegal.
Prefix 'un-' (meaning 'not') combined with 'lawfully' (from Old English 'lagu' meaning law, plus '-ful' and '-ly' for manner). The construction is straightforward: not in a lawful manner.
English is full of these prefix combinations (unlawfully, unjustly, unfairly) that let us negate entire concepts with just three letters, which is why many legal documents have such dense, nested negations like 'unlawfully and without just cause'—the prefixes stack up.
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