Chock-a-block means crammed full or crammed tightly together and is originally a nautical expression dating from the early 19th century. It is sometimes shortened to ‘chocka’ or ‘chocker’, which usage dates from the mid-20th century. When two block and tackles or pulleys came together after hoisting loads aboard ship, they were described as being chock-a-block or crammed together. By the mid-19th century, chock-a-block was being used widely rather than in a specific nautical context. The ‘chock’ part of the expression derives from chockfull meaning filled with no available space, which expression is a a great deal older. Some sources say from as long ago as the 1400s but the OED says the 17th century is more likely.