means The seafloor is the solid bottom of an ocean, sea, or other large body of water — everything beneath the waves that you could, in theory, stand on.

from A plain English compound, bolted together from two old workhorses. "Sea" descends from Old English sǣ, with Germanic cousins all across the North Sea coast — Dutch zee, German See. "Floor" comes from Old English flōr, meaning the ground underfoot, related to words for a flat surface or plain across the Germanic and Celtic worlds. Put them side by side and you get exactly what it says: the floor of the sea. The term is fairly modern as a single word — the deep ocean bottom was, for most of history, an abstraction no one had the means to see, so naming it had to wait for the instruments that could reach down and touch it.