Definition of Ice Water In Their Veins

What does the term "ice water in their veins" mean in the world of sports? What is meant by "ice water in their veins"?

"Ice water in their veins" is referring to somebody who is thought to be cool under pressure.

Some performers get anxious and make mistakes when they are under pressure.

If somebody is said to have "ice water in their veins", they are thought to operately calmly under pressure.

An example?

Tom Brady, quarterback of the Tampa Bay Buccaneers (formerly the New England Patriots), who has excelled under the brightest of lights many times over the course of his career.

Brady is said to have "ice water in his veins", as the rising pressure of a Super Bowl or other big game doesn't seem to have any impact on his performance.

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The opposite of somebody having ice water in their veins is a player that makes mistakes when the lights are their brightest, such as fumbling at a key point of the game or letting in a soft goal.