Even brave sailors fear rock-caved Charybdis“Incidit in Scyllam cupiens vitare Charybdim!”. In 'The Odyssey' Homer describes Skylla, or Scylla, as a barking-mad monstrous goddess who together with the whirlpool daemon Charybdis makes life hell for sailors and their ships in the Strait of Messina (between Sicily and Calabria, Italy). Any ship sailing too close to the sharp-toothed Skylla would lose six sailors, one for each of her six heads, whereas anyone sailing near Charybdis, who lives on the other side of the narrow strait, would risk being ship wrecked in her rough waters.
Who drinks the waves, vomits them out again,
And Scylla with her barking dogs around her
Churning the waves that circle Sicily.
Thus when I shun Scylla, your father, I fall into Charybdis, your mother (Merchant of Venice Act III, Sc. V)
Scylla and Charybdis are two rocks located in a narrow sea passage in the Straits of Messina. Scylla is located on the Italian side, while Charybdis is on the Sicilian side.
In ancient Greek mythology, Scylla was a female monster with twelve feet and six heads. She had razor sharp teeth and was capable of ripping apart sailors who had the misfortune of coming too close to her. Charybdis was a whirlpool. It was seen as a monster, which gulped down huge amounts of water and in the process sucked in sailors who accidentally got close to it.
When Ulysses tried to make his way through this narrow passageway, Scylla managed to kill six of his sailors. When you say you are between Scylla and Charybdis what you are implying is that you are between the devil and the deep blue sea. You are being threatened by two dangers at the same time and in trying to avoid one you fall victim to the other.
The two dangers are often seen as representing life. Trying to avoid one mistake, we often end up making another.
Possibly related to 'between the Devil and the deep blue sea'.
"I, with my partie, did lie on our poste, as betwixt the devill and the deep sea."