
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.

rear / back part
The rear area or back side of an object, vehicle, or building.
Das Hinterteil des Busses war mit Graffiti beschmiert.
The rear of the bus was smeared with graffiti.
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.
ass / asshole
Arsch and Hinterteil can both refer to a person's buttocks. Arsch is strongly vulgar and often sounds especially coarse or emotional. Hinterteil is more neutral and is suitable when speaking about the same body part in an objective or polite way.

Nach dem Sturz erklärte der Mann ruhig, dass ihm das Hinterteil wehtat.
After the fall, the man calmly explained that his backside hurt.
Nach dem Sturz fluchte der Mann laut, dass ihm der Arsch wehtat.
After the fall, the man loudly swore that his ass hurt.