1. Marked by or showing lack of due forethought or care; careless: a thoughtless mistake; a thoughtless bumbler. See Synonyms at careless.
2. Inconsiderate of the feelings of others: a thoughtless remark.
3. Lacking thought: staring blankly in a thoughtless daze.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.