Why is the side of the road called the shoulder?

shoul·der
ˈSHōldər/Submit
noun
1. the upper joint of the human arm and the part of the body between this and the neck.
2. a part of something resembling a shoulder in shape, position, or function.
"the shoulder of a pulley"
verb
1. put (something heavy) over one's shoulder or shoulders to carry.
"we shouldered our crippling backpacks and set off slowly up the hill"
2. push (someone or something) out of one's way with one's shoulder.
"she shouldered him brusquely aside"
synonyms: push, shove, thrust, jostle, force, bulldoze, elbow
"another kid shouldered him aside"

You get this screen by typing "define shoulder" into the address window of your browser. You can also go to google.com and type it in the dialog box. If you do this a lot you will become really smart.

It depends on your point of view. If you imagine you are looking along the roadway and seeing the shape of the dirt, you would call it a shoulder. If you were in the field looking at a right angle to the roadway you would call it a bank or sometimes an embankment.