It's thought that the expression had some currency in the era of the fur traders, when an experienced beaver trapper was said to be able to read all kinds of things from the way in which his float stick (attached by length of cord to the trap) lay on the surface the water.
Alternative Old West expressions include: Know which way is up; know your ass from a hole in the ground; and my personal favourite, know poor bull from fat cow.
'Know what's what' would work for us sons of the city (greeners, junipers, gunsels, pilgrims, tenderfeet...)
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