careless

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Taking insufficient care; negligent.
• adjective Marked by or resulting from lack of forethought or thoroughness.
• adjective Showing a lack of consideration.
• adjective Unconcerned or indifferent; heedless.
• adjective Unstudied or effortless.
• adjective Exhibiting a disposition that is free from cares; cheerful.

from The Century Dictionary.

• Free from care or anxiety; hence, undisturbed; cheerful.
• Giving no care; heedless; negligent; unthinking; inattentive; regardless; unmindful.
• Done or said without care; unconsidered: as, a careless act; a careless expression.
• Not receiving care; uncared for.
• Synonyms and Supine, Indolent, etc. (see listless); incautious, thoughtless, remiss, forgetful, inconsiderate.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Free from care or anxiety. hence, cheerful; light-hearted.
• adjective Having no care; not taking ordinary or proper care; negligent; unconcerned; heedless; inattentive; unmindful; regardless.
• adjective Without thought or purpose; without due care; without attention to rule or system; unstudied; inconsiderate; spontaneous; rash.
• adjective rare Not receiving care; uncared for.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective marked by lack of attention or consideration or forethought or thoroughness; not careful
• adjective effortless and unstudied
• adjective (usually followed by `of') without due thought or consideration