From Glossary of Meteorology
The side of a tropical cyclone to the right of the direction of movement of the storm in the Northern Hemisphere (to the left in the Southern Hemisphere), where the winds are stronger because the cyclone's translation speed and rotational wind field are additive.
The opposite side is termed the navigable semicircle. This terminology originated in the days of sailing ships. It occurred naturally since 1) the dangerous semicircle of the storm has the strongest winds and heaviest seas; 2) a sailing ship on this side tends to be carried into the path of the storm; and 3) if the storm recurves, its center is likely to cross the course of a ship running before the wind.