Originally Nautical (later also Aeronautics). Of the wind, etc.: favourable to the course of a ship, aircraft, etc. Also figurative and in…

esp. of wind, weather, etc. (Here there is contact with sense A. 5)

Attended with advantage or convenience; facilitating one's purpose or wishes; advantageous, helpful, suitable. Said esp. of the weather, etc.

Promoting or conducing to success; bringing prosperity; favourable, auspicious, propitious. Now archaic.

Nautical. Of wind: Too much ahead, so that the ship has to sail very close. Cf. scant, v. 2 (Opposed to large or free.)

Of the wind: Blowing across the direct course, contrary. Also figurative. (See cross-wind, n.)

Nautical. Of winds: That blow about and make straight sailing impracticable; shifting, variable.

Of a person, action, agent, or force: acting against or in opposition (to someone or something); opposing, antagonistic, actively hostile.

Of wind or tide: moving in the same direction as the course of a ship, aeroplane, or other object in motion.