Definition: (adjective) Characterized by undue haste and lack of thought or deliberation.
Synonyms: brainish, impetuous, tearaway, hotheaded, impulsive.
Usage: This madcap quest of mine, was it not understood between us…to be a fantastic whim, a poetical wild-goose chase, conceived entirely as an excuse for being some time in each other's company?
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Source: The Free Dictionary