A pole is called a place that is for some reason the furthest away or has extreme characteristics. The poles (north and south) are those places on Earth that are their absolute opposites - the northernmost area of the northern hemisphere and the analogous area in the southern hemisphere. If something is described as bipolar then we can certainly expect a wide range of phenomena. The word 'pole' means the extreme, the maximum intensity of a feature. This is exactly how one would look at BPAD - a constant manoeuvring between affective extremes, from depression to a state of mania.