noun, masculine/a.ʁjɛʁ.tʁɛ̃/plural: arrière-trains
the rear of a train, or 4x4 vehicle; hindquarters (of an animal); caboose, posterior, hindquarters (buttocks of a human)
“Le chien[…] n'avait pas de queue, alors il remuait l'arrière-train pour signifier qu'il était content.” — The dog […] didn't have a tail, so it wagged its hindquarters to show it was happy.
From arrière- + train.