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Someone with a normal temperature, ie, not a fever.
Kid: "I think I might have a fever... I'm really feeling sick."
Kid's mom: "Let me feel your forehead." (pause) "Nope, cool as a cucumber."
by bandcampgirl183 October 02, 2005
(adj.) Cool with the added simile of a watery vegetable.
He was as cool as a cucumber.
by Gumba Gumba June 01, 2004