thor·ough (thûrō, thŭrō)
1. Exhaustively complete: a thorough search.
2. Painstakingly accurate or careful: thorough research.
3. Absolute; utter: a thorough pleasure.
prep. & adv.
Variant of through.
[Middle English thorow, through, thorough, from Old English thuruh, from end to end, through; see terə-2 in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 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.