das Hinterteil
backside
A person's buttocks or the rear part of an animal's body.
Das Kind ist beim Spielen auf sein Hinterteil gefallen.
The child fell on his backside while playing.

backside / rear
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backside
A person's buttocks or the rear part of an animal's body.
Das Kind ist beim Spielen auf sein Hinterteil gefallen.
The child fell on his backside while playing.

The word is a clear compound of 'hinter' (behind) and 'Teil' (part). So it literally refers to the part that is at the back.
Der Hund wedelte mit seinem ganzen Hinterteil.
The dog wagged its whole rear end.
You can always tell which meaning is intended from the context. If people are talking about a person or an animal, it means the buttocks or rear end. If they are talking about an object, it means its back side.
Er hat sich beim Sport am Hinterteil verletzt.
He injured his backside while playing sports.
In the meaning 'Gesäß' (buttocks), 'Hinterteil' (backside) is often used as a somewhat childish or mildly formal paraphrase. It is not as direct as 'Arsch' (ass/arse) and not as medical as 'Gesäß' (buttocks).
Der kleine Junge bekam einen liebevollen Klaps auf das Hinterteil.
The little boy got an affectionate pat on the backside.
butt
'der Hintern' and 'das Hinterteil' can both mean the buttocks. However, 'Hintern' is clearly colloquial and fits more in relaxed everyday situations. 'Hinterteil' sounds more neutral and can also mean the rear part of animals or things, for example a car. The words can be confused because, with people, they often refer to the same body part. If you want to sound friendly or more factual, 'Hinterteil' is usually the more careful choice. 'Hintern' sounds more direct and sometimes a bit crude.

Das Hinterteil des Autos war nach dem Unfall stark beschädigt.
The rear of the car was badly damaged after the accident.
Nach der langen Radtour tat mir der Hintern weh.
After the long bike ride, my butt hurt.