1. A large clasp knife.
2. Sports A dive in the pike position, in which the diver straightens out to enter the water hands first.
v. jack·knifed, jack·knif·ing, jack·knifes
1. To fold or double (something or oneself) like a jackknife.
2. To cut or stab with a jackknife.
1. To bend or fold up like a jackknife: A truck that had jackknifed was blocking the road.
2. To form a 90° angle.
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.