In a manner that violates or breaks the law. Done without legal authorization or in contravention of established legal rules.
From Latin 'illegalis' (not legal), formed by combining the prefix 'il-' (not) with 'legalis' (legal), itself from 'lex/legis' (law). The adverbial '-ly' suffix was added in English to create the manner adverb.
The concept of 'illegally' is fascinating because what's illegal in one jurisdiction may be perfectly legal in another, highlighting how laws are cultural constructs rather than universal truths. The word also raises philosophical questions about civil disobedience - when breaking the law might be considered morally justified.
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