This is the 9450th most frequent French word.
dos
Back.
Here, 'dos' refers to the anatomical structure, indicating a part of the body.
J'ai mal au dos.
My back hurts.
In this sentence, 'dos' describes the back as a location where the bag is carried.
Il porte son sac sur le dos.
He is carrying his bag on his back.
This usage of 'dos' specifies the body's posture while lying down, pointing to the back as a surface.