The phrase “grace under pressure” is Ernest Hemingway's: Wikiquote gives it from a letter to F. Scott Fitzgerald of 20 April 1926, published in his Selected Letters (1981), and Dorothy Parker used it in her 1929 New Yorker profile of him. In Parker's profile Hemingway gives “grace under pressure” as his definition of guts. The five-word sentence beginning “Courage is” is not in either source as they are quoted.
What it means
Keeping your poise when things are hardest: behaving well, even elegantly, at exactly the moment it is most difficult to.
Where it comes from
According to Wikiquote, Hemingway used the phrase in a letter to Fitzgerald of 20 April 1926, published in Ernest Hemingway: Selected Letters 1917–1961 (1981), edited by Carlos Baker, and in it wrote that he was “not referring to guts but to something else.” Dorothy Parker then used it in her profile of Hemingway, “The Artist's Reward”, in the New Yorker of 30 November 1929, in an exchange Wikiquote gives as: “Exactly what do you mean by ‘guts’?” “I mean,” Ernest Hemingway said, “grace under pressure.”
So the phrase was Hemingway's, and in Parker's profile it was his definition of guts. The sentence “Courage is grace under pressure” is a later tidying of that exchange: neither source, as Wikiquote quotes them, has Hemingway saying it in those five words. We have not seen the letter or the magazine ourselves, so this label reflects only that the five-word sentence has no source.