If a place is chock-a-block, it is so full of people or things that you can hardly add one more. A train at rush hour, a suitcase, a week of meetings — all can be chock-a-block. It is informal, and you hear it more in Britain than in America. You may also see it written chockablock. Though actually, the tidy picture is a ship: sailors pulled a rope through two pulley blocks until the blocks sat tight against each other, with no gap left. People then borrowed that picture for anything packed to the limit. It does not mean a block of chocolate, and it is not the wheel-block word chock used on its own.