means A small boat propelled by an engine rather than by sails or oars.

from A transparent compound: "motor" plus "boat," stitched together in the early days of the internal-combustion engine when small craft first traded canvas and oars for machinery. "Motor" comes from the Latin movere, "to move" (by way of motor, "mover"), the same root that gave us "motion" and "momentum" — so a motorboat is, quite literally, a moving-boat.