Harebrainedly is an adverb that means foolishly or without careful consideration. Synonyms for harebrainedly include recklessly, thoughtlessly, imprudently, impulsively, carelessly, rashly, heedlessly, improvidently and unwisely. These synonyms are useful when describing someone or something that is acting in a foolish or reckless way. For example, someone who makes a spur-of-the-moment decision without any forethought could be described as acting harebrainedly. Similarly, a business decision that is made without careful consideration of the consequences could also be described using one of these synonyms. Overall, these words are useful when you want to convey the idea of acting without caution or common sense.