What does the idiom “between Scylla and Charybdis” mean?

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Scylla and Charybdis were situated in close proximity on either side of a strait, meaning that no ship could pass without encountering one or the other or both. To avoid Scylla a ship would have to risk the whirlpool, and to avoid Charybdis it would have to venture near Scylla. The impossible choice originated the phrase “between Scylla and Charybdis,” which means to be caught between two equally unpleasant alternatives.