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also air-line, 1813, "beeline, straight line from place to place" (as through the air, rather than over terrain; in later 19c. especially in reference to railways that ran directly between big cities in the U.S. instead of meandering from town to town in search of stock subscriptions as early railways typically did), from air (n.1) + line (n.). Meaning "public aircraft transportation company" is from 1914.