E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 windi.e. wind of a specific beat or tread; tredan, to tread.)