[hgsgm]

> and a rumble seat

What?

[theodric]

When your butt's so big it fits two extra people https://en.m.wikipedia.org/wiki/Rumble_seat

[karmelapple]

It's a humorous phrase members of my family have used for, as others have mentioned, a larger posterior.

[red-iron-pine]

he dun got a big butt

[Arrath]

Junk in the trunk