Jumpsuit

In 1941 World WarⅡ, US Army, two piece parachutist uniform designed by William P. Yarborough that made a reappearance in 1963 as the Army tropical combat uniform. and later became the pattern of the US military Battle Uniform. by Wikipedia
Zipper from neck until crouch for convenience reson.
Original purpose for Jumpsuit is for hard condition worker to keep the body clean. Two separate cloth might get dirty from the waist part.