Taking insufficient care; negligent: a careless housekeeper; careless proofreading. Marked by or resulting from lack of forethought or thoroughness: a careless mistake. Showing a lack of consideration: a careless remark. Unconcerned or indifferent; heedless: careless of the consequences. Unstudied or effortless: danced with careless grace. Exhibiting a disposition that is free from cares; cheerful: a careless grin; a careless wave of the hand.