The reason why they call it shiplap is because shiplap uses the overlapping technique like on ships called lapping.
The overlapping boards that are used in shiplapping is how shiplap got it's name.
The overlapping technique known as lapping on ships is what creates a watertight seal.
Shiplap was originally used for siding on the exterior of hones, barns and sheds and it was used as a protective covering between the framing and exterior.
Shiplap is also sued in interiors of homes as a horizontal plank which overlaps with other boards and can also be used to create a rustic finish and look.
Shiplap was also sued as sheathing between framing and the exterior before Sheetrock.
Shiplap is strong and can form a tight seal and can be attached horizontally and vertically and horizontal boards can make a room feel even larger and vertical boards can make a low ceiling seem higher as well.
Shiplap also became very popular in the 1930s in the United States and it was also easy to mill, install and transport.
Basically Shiplap is derived from it's original purpose in the use of shipbuilding.
Vikings were also known for their advanced maritime technology and included using wooden planks with a rabbet joint along each edge that allowed them to create watertight vessels that were capable of navigating rough seas without taking on water.