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A large clasp knife.
(noun)
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Sports A dive in the pike position, in which the diver straightens out to enter the water hands first.
(noun)
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To fold or double (something or oneself) like a jackknife.
(verb-transitive)
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To cut or stab with a jackknife.
(verb-transitive)
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To bend or fold up like a jackknife: A truck that had jackknifed was blocking the road.
(verb-intransitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.