The term 'ACHILLES WAS DIPPED IN IT' is referring to the river Styx, which is a four-letter word. In Greek mythology, the river Styx was a boundary between the Earth and the Underworld. It was believed that if a living being was dipped in the river Styx, they would become invulnerable. This is what happened to Achilles, a hero of the Trojan War, when his mother dipped him in the river Styx as an infant.