This phrase is often used to describe a surgery or sugerical process. Knife standing for “surgery” was first recorded in 1880.
Example:
Sam: "When do you go under the knife?

Tim: "I have surgery next Wednesday. I have to take an

Ativan before I get there."
by SigmaNutz December 25, 2011
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"Jose, I'm so nervious about your brain condition, I think you'll have to go under the knife."
by Ross March 29, 2005
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Shaved the pubic region with 1) a razor 2)brother's electric razor 3) dad's electric razor
Brian Keehner told us he went under the knife... but *someone* else hasn't... hmmmm
by Angie Groves January 9, 2004
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