Reference > Brewer’s Dictionary > Cool Card.

 Cooks.Cool as a Cucumber. 
Cool Card.
You are a cool card (or pretty cool card). A person who coolly asks for something preposterous or outrageous. Card = character, hence a queer card, a rum card, etc. And “cool” in this connection means coolly impudent.   1
   Gifford says the phrase means a “cooling-card, or bolus”; but this is not likely, as a cool-card acts generally as an irritant. A person’s card of address is given at the door, and represents the person himself, and this without doubt is the card referred to.   2
        “You’re a shaky old card; and you can’t be in love with this Lizzie.”—Dickens Our Mutual Friend, book iii. chap. i. p. 192.

 Cooks.Cool as a Cucumber. 

