The orlop is the lowest deck

Deck (ship)

A deck is a permanent covering over a compartment or a hull of a ship. On a boat or ship, the primary deck is the horizontal structure which forms the 'roof' for the hull, which both strengthens the hull and serves as the primary working surface...

 in a ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

 (except for very old ships). It is the deck or part of a deck where the cables are stowed, usually below the water line. It has been suggested the name originates from "overlooping" of the cables.

It has also been suggested that the name is a corruption of "overlap", referring to an overlapping, balcony-like half deck occupying a portion of the ship's lowest deck space. According to the Oxford English Dictionary, the word descends from Dutch

Dutch language

Dutch is a West Germanic language and the native language of the majority of the population of the Netherlands, Belgium, and Suriname, the three member states of the Dutch Language Union. Most speakers live in the European Union, where it is a first language for about 23 million and a second...

overloop from the verb overlopen, "to run (over); extend").