Pronunciation: (un-thing'king),[key] —adj. 1. thoughtless; heedless; inconsiderate: an unthinking, tactless person. 2. indicating lack of thought or reflection: a dull, unthinking expression on his face. 3. not endowed with the faculty of thought: unthinking matter. 4. not exercising thought; not given to reflection: This book will not profit an unthinking person. 5. not thinking; unmindful: chores done in an unthinking manner.