The part that supplies the motion everything else depends on. An engine does the work; the rest directs it.
Old French engin (skill, cleverness, contrivance), from Latin ingenium (natural talent), in- plus gignere (to beget, produce).
Its root is cleverness, not brute motion: ingenium named a natural talent, the same root behind ingenious and genius. An engine was, for most of the word's life, a clever device before it ever became the part that merely supplies force.