Phrase Finder suggests this phrase plays on the two meanings of “cool” as a low temperature (cucumbers are cool to the touch) and imperturbable. The phrase was first recorded in a poem in 1732. The Online Etymology Dictionary adds that the idea that cucumbers feel cool was “confirmed by science in 1970: inside of a field cucumber on a warm day is 20 degrees cooler than the air temperature.” I am not surprised that a poet would create a memorable phrase, but for “cool as a cucumber” to last for nearly 300 years with its meaning intact is quite an accomplishment.

Wordwizard.com has several explanations from different sources for this expression, which is used to describe defeating someone decisively. As William Safire explained in the New York Times, the phrase begins with “clean,” which was applied to the victims of thieves or gamblers who had been drubbed, defeated, and/or wiped out. The “clock” probably comes from the fact a clock has a face, and “clock him one,” refers to hitting him in the head. Brewer’s Dictionary of Modern Phrases & Fable has a similar explanation saying that the expression has a military origin from World War II, and that, “A person’s face is probably their face…” Cassell’s Dictionary of Slang speculates it might be railroad jargon referring to applying the airbrakes and bringing the train to a sudden stop. “The ‘clock” in question is the air gauge, which on halting, immediately registers zero and is thus ‘clean’.”