The line: "My salad days, when I was green in judgment: cold in blood, To say as I said then!” comes from which play of William Shakespeare?

Historians, literary experts, and others have stated that this metaphor (phrase) was coined in Shakespeare's play "Antony and Cleopatra" in 1606. In the speech at the end of Act I, Scene V (lines 607-609), Cleopatra is regretting her youthful dalliances with Julius Caesar. She specifically uses the phrase: "My salad days, when I was green in judgment: cold in blood, To say as I said then!”

In this case, Cleopatra's use of the word 'green' — presumably meaning someone youthful, inexperienced, or immature. Her references to "green" and "cold" both suggest qualities found in salads.

In more modern times, especially when the United States is mentioned in books, newspapers, and other writings, the metaphor used by Shakespeare refers to a heyday, a period when somebody was at the peak of their abilities. The person did not necessarily have to be in their youth.