underpart

[ uhn-der-pahrt ]

/ ˈʌn dərˌpɑrt /

the lower part or side: The underpart of the plane's fuselage scraped the treetops.

an auxiliary or secondary part or role.

Origin of underpart

First recorded in 1655–65; under- + part

We had the underpart of the slab on our side, and I did not think of looking when we took it down.

On the Irrawaddy |G. A. Henty
A pearly radiance seems to strike upwards from the sea on to the underpart of the clouds, which borrows an abnormal glow.

A Floating Home |Cyril Ionides
The top, sides, and underpart are all of bulletproof construction.

Warren Commission (5 of 26): Hearings Vol. V (of 15) |The President's Commission on the Assassination of President Kennedy
Springing in a slow, graceful curve from the underpart of the body over the forecarriage is a splasher of crimson patent leather.

Carriages & Coaches |Ralph Straus

The underpart of the foot forms a tube called the funnel (or siphon).

Zoology: The Science of Animal Life |Ernest Ingersoll