Other forms: hairlines
The hairline is the edge where the hair begins on the forehead. A receding hairline, a high hairline, the hairline moved back.
Male pattern baldness starts there and at the crown, and it is driven by hormones acting on follicles that are genetically sensitive. A hairline crack is a different sense — one as fine as a hair, and in bone or metal it can still be serious.
1nounhairlineThe edge where the hair begins on the forehead.
“His hairline had begun to move back, so he decided to shave his head.”
hairline is an edge.
Hair plus line. The second sense, a very fine crack, comes from the width of a single hair.
From hair + line.
Etymology from Wiktionary, CC BY-SA