wack·o (wăkō) also whack·o (wăkō, hwăkō) Slang
n. pl. wack·os
A person regarded as eccentric or irrational: "a catchy pop portrait of a wacko who talks to himself in French" (Phoebe Hoban).
[Alteration of WACKY.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. 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.