英语词源字典 - 在线词源网站

airline (n.)[airline 词源字典]: also air-line, 1813, "beeline, straight line between two points on the earth's surface" (as through the air, rather than over terrain; from 1853 and 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.[airline etymology, airline origin, 英语词源]
hairline (n.): also hair-line, "cord made of hair," 1731, from hair + line (n.). Meaning "a very fine line" is from 1846. As "the outline of the hair on top of the head," by 1903. As an adjective, of cracks, etc., 1904.