On the weather side; toward the wind, as opposed to the lee side of a ship or structure.
From a- (on, in a state of) + weather (Old English weder, 'wind, storm'). This is primarily a nautical term that developed as sailors needed precise directional language for safety at sea.
Sailors had dozens of words for positioning relative to wind because getting it wrong meant drowning—'aweather' is just one of many directional terms that governed life and death on the ocean.
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