Why is the ninth month called September?
"cord made of hair," 1731, from hair + line. 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., by 1923.
hairline hair·line (hâr'līn')n. The outline of the growth of hair on the head, especially across the front.