Stuck in a situation where you face two dangers on either side of you, and moving away from one danger would be moving closer to the other.
Taken from
Homer's Odyssey, in which
Odysseus and his men must sail in between two ancient sea-monsters/Gods.
"If I
tell her I like her, she'll get too
clingy, but if I don't tell her anything, she might think I'm not interested. I'm really
between Scylla and Charybdis here"