In Greek mythology, Charybdis (or Kharybdis) was once a beautiful naiad and the daughter of Poseidon and Gaia. She assumes the form of a huge bladder of a creature whose face is all mouth and whose arms and legs are flippers. She swallows a huge amount of water three times a day, before belching it back out again, creating large whirlpools capable of dragging a ship underwater. In some variations of the story, Charybdis is simply a large whirlpool instead of a sea monster. Once a lovely maiden, Charybdis was loyal to her father in his endless feud with Zeus. She rode the hungry tides after Poseidon stirred up a storm, directing them onto beaches, destroying entire villages, submerging fields and drowning forests, claiming all in her path for the sea. She claimed so much land for her father's kingdom that Zeus became enraged and changed her into a monster.
In mythology Charybdis lies on one side of a narrow channel. Opposite her is Scylla, another sea-monster. The sides of the strait are within an arrow shot of each other, and sailors attempting to avoid one of them will come in reach of the other. 'Between Scylla and Charybdis' thus means to having to choose between two dangers, either of which brings harm.