If you can't stand the heat, get out of the kitchen. Don't persist with a task if the pressure of it is too much for you. The implication being that, if you can't cope, you should leave the work to someone who can.
From one plain speaker to another... We're Wild About Harry now more than ever before.
This is widely reported as being coined by US President Harry S. Truman. That's almost correct, but in fact, Truman was known to have used it at least as early as 1942 - before becoming president. Here's a citation from an Idaho newspaper The Soda Springs Sun, from July that year:
"Favorite rejoinder of Senator Harry S. Truman, when a member of his war contracts investigating committee objects to his strenuous pace: 'If you don't like the heat, get out of the kitchen'."
He used a version slightly nearer the one most often used nowadays, in 1949, after becoming president, when warning his staff not to concern themselves over criticism about their appointments:
"I'll stand by [you] but if you can't take the heat, get out of the kitchen."
Truman was well-known as a plain-speaker, in a way that politicians in our more media-sensitive age rarely are. This was celebrated by Merle Miller, who published a set of interviews with him - called Plain Speaking: An Oral Biography of Harry S. Truman, in 1974. It includes this unambiguous gem, which would hardly get past the presidential spin-machine these days:
"I didn't fire him [General MacArthur] because he was a dumb son of a bitch, although he was, but that's not against the law for generals. If it was, half to three-quarters of them would be in jail."
The other phrase associated with Truman that has entered our language is 'the buck stops here'. It was a sign, which sat on the president's desk.
The saying "the buck stops here" derives from the slang expression "pass the buck" which means passing the responsibility on to someone else. The latter expression is said to have originated with the game of poker, in which a marker or counter, frequently in frontier days a knife with a buckhorn handle, was used to indicate the person whose turn it was to deal. If the player did not wish to deal he could pass the responsibility by passing the "buck," as the counter came to be called, to the next player.*
On more than one occasion President Truman referred to the desk sign in public statements. For example, in an address at the National War College on December 19, 1952, Mr. Truman said, "You know, it's easy for the Monday morning quarterback to say what the coach should have done after the game is over. But when the decision is up before you -- and on my desk, I have a motto which says The Buck Stops Here' -- the decision has to be made." In his farewell address to the American people given in January 1953, President Truman referred to this concept very specifically in asserting that, "The President--whoever he is--has to decide. He can't pass the buck to anybody. No one else can do the deciding for him. That's his job.
The sign has been displayed at the Library since 1957.