vast expanse of water; unfathomable; incoherent
Literary and archaic noun. Describes a boundless body of water, or metaphorically something deep, vast, and hard to grasp. The 'incoherent' sense is rare and may refer to rambling speech or writing.
In classical texts, it is used like 'the vast, boundless ocean'.
His argument was unfathomable and hard to pin down.