Hell was the underworld in some ancient Human religions, typically ruled by a figure called the Devil. It was believed people would be sent here after their death, if they did not adhere to the religion's code.
In 2267, Matthew Decker considered that the Planet killer came "Right out of Hell". (TOS: "The Doomsday Machine") That same year, Lazarus had proclaimed that he would chase his counterpart, the anti-Lazarus, to the very "Fires of Hell". (TOS: "The Alternative Factor")
The word "Hell" is often used in Terran vernacular as an epithet, as in an exclamation that Captain James T. Kirk used at least once; "Let's get the Hell out of here." (TOS: "The City on the Edge of Forever")