Brewer's: Trade Winds
Winds that trade or tread in one uniform track. In the northern hemisphere they blow from the north-east, and in the southern hemisphere from the south-east, about thirty degrees each side of the equator. In some places they blow six months in one direction, and six in the opposite. It is a mistake to derive the word from trade (commerce), under the notion that they are ?good for trade.?
(Anglo Saxon, tredde-wind, a treading wind?i.e. wind of a specific ?beat? or tread; tredan, to tread.)