(しり)

しり
noun
buttocks, bottom, rear
See also: (しり) (polite form)
1. buttocks, bottom
The posterior part of the body that one sits on.
My bottom hurts from sitting in a chair too long.
I wiped the baby's bottom.
(しり) is the polite form
He's restless and changes jobs quickly.
(しり)(かる) = restless, fickle
2. rear end, back part
The back or trailing part of something.
I lined up at the back of the queue.
There's a stain on the back part of the skirt.
The heels of my shoes are worn down, so I need to buy new ones.

Can be considered slightly crude in formal contexts. Alternative polite forms exist.

COMMON EXPRESSIONS:

POLITE ALTERNATIVES: