E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.
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 persons card of address is given at the door, and represents the person himself, and this without doubt is the card referred to.
Youre a shaky old card; and you cant be in love with this Lizzie.Dickens Our Mutual Friend, book iii. chap. i. p. 192.