the main character in a story called Ali Baba and the Forty Thieves, from TheArabian Nights. Ali Baba sees 40 thieves enter a secret cave (=a large hole in the side of a cliff or under the ground) by saying the magic words "Open Sesame", which makes a door in the cliff open. He repeats these words, enters the cave, and takes some of the gold that the thieves keep there.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.
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