In the poker variant of Texas Hold 'em, Big Slick refers to holding an ace and a king as your two hole cards. It is famously refered to Big Slick as it is a slippery hand that one can easily lose a fortune over. Usually grouped together as one of the premium hands along with Aces and Kings, but this hand is more flexible especially if suited.
Other nicknames for Big Slick include: Space Cowboy, Walking Back to Houston, Big Ugly, Santa Barabara, Ass Kicker, Machine Gun, and Big Slick in a Suit (if suited)
From Urban Dictionary