In a manner showing lack of regard, respect, or proper attention; in a disrespectful or careless way.
From disregardful + -ly (adverb-forming suffix from Old English 'like'). The adverb allows this adjective to modify verbs and other adjectives.
Adding '-ly' to an already complex word like 'disregardful' creates a mouthful that almost no one uses—instead people say 'carelessly' or 'recklessly', showing how English gradually eliminates unnecessarily long synonyms.
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