"At either stock or wall"
Beds in cold countries were often heavily curtained or partially enclosed. There sometimes was a canopy (tester) suspended over the bed, and heavy bed curtains all around, to retain the body heat of the occupants. When the bed could be placed head to the wall, which requires a fairly large bedroom, then both sides of the bed would be accessible. If the bed has to be placed against the wall, however, as it must be in the typically small room used as a private bedroom, then one sleeping spot is against the wall and one is on the outside. Typically, the man slept on the outside so that he could respond in an emergency, see the door, and reach his weapons, and the woman slept on the inside, where it was warmest. I haven't found a reference for calling the outside the stock, but I speculate that the outside was called the 'stock' because it was often lined with chests for valuables and clothes, which served as a step up to what was usually quite a high bed platform, and they provided a place for visitors to sit--people received visitors in their bedrooms more freely when houses were smaller.

In Scotland this tradition was taken to the extreme of building a wooden box accessed on one side only, at the head. These box beds were common in Scotland from the 14th century, but are part of a much older nordic tradition. You can see the remains of a stone box bed built right into the wall at one of the excavated houses at Scara Brae.