(~s)
1.
A ~ is a long thin vegetable with a hard green skin and wet transparent flesh. It is eaten raw in salads.
N-VAR
2.
If you say that someone is as cool as a ~, you are emphasizing that they are very calm and relaxed, especially when you would not expect them to be.
You can hardly be held responsible for Darrow waltzing in, cool as a ~, and demanding thousands of pounds.
PHRASE emphasis