Ice man

From Memory Alpha, the free Star Trek reference.

Ice man is a colloquial term given to someone who is unusually calm and focused under pressure or someone who lives or originates in an icy or arctic region.

While playing poker, Commander Riker called Lt. Worf "ice man" after winning at least two consecutive hands of the game. (TNG: "The Measure Of A Man")

After suffering amnesia from a shuttle accident, Talur, a native of Barkon IV erroneously claimed Lieutenant Commander Data an ice man because of his pale physical features. (TNG: "Thine Own Self")

The Klingon term for "ice man" is d'akturak. Federation Ambassador Curzon Dax gave this nickname to Koloth at the Korvat colony negotiations, referring to Koloth's "icy" demeanor at those negotiations. (DS9: "Blood Oath")