Completely full or tightly packed; crammed so full that there's no space left.
From nautical terminology 'chock' (a block or wedge) + 'block.' When two blocks meet in a pulley system with no space between them, the result is 'chock-a-block.' The term was extended to mean 'completely full.'
Sailors invented this term for a mechanical condition on ships, but it perfectly described their cargo holds—so the word jumped from technical jargon to everyday speech to describe any overstuffed space.
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