1. Marked by lack of attention, consideration, forethought or thoroughness; not careful
   "forgotten by some careless person"; "a careless housekeeper"; "careless proofreading"; "it was a careless mistake"; "hurt by a careless remark"; "careless about her clothes";
   - cavalier
    
2. Effortless and unstudied
   "an impression of careless elegance"; "danced with careless grace"
    
3. (usually followed by ‘of’) without due thought or consideration
   "careless of the consequences";
   - regardless

See also: artless, carefulness, casual, cursory, haphazard, heedless, imprudent, inattentive, incautious, negligent, offhand, offhanded, passing, perfunctory, reckless, slapdash, slipshod, sloppy, superficial, unattentive [rare], unconcerned, unheeding

Antonym: careful

Encyclopedia: Careless, William