in a situation where you have to choose between two possible actions, but both are dangerous or unpleasant. The phrase comes from the ancient Greek stories of Scylla, a monster, and Charybdis, a whirlpool (=water that spins around and pulls things down into it), that killed sailors in the sea between Italy and Sicily. People sometimes also use the phrase "between a rock and a hard place" to mean the same thing.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.
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