(attributive) designating a part which first receives motion from an engine or motor and communicates it to another part; specifically designating the input shaft of a gearbox.
Origin
Mid 19th century.
Definition of first motion in US English:
first motion
adjective
Mechanics
(attributive) designating a part which first receives motion from an engine or motor and communicates it to another part; specifically designating the input shaft of a gearbox.