also sky-line, "horizon; place where the sky and the earth, or objects on it, seem to meet," 1824, from sky (n.) + line (n.). Originally often in reference to paintings.

1785, "line marked on a surface before cutting," from guide + line (n.). Meaning "rope for steering a hot-air balloon" is from 1846. In figurative use by 1948. Related: Guidelines.

late 14c., "stuff with which garments are lined," verbal noun from Middle English linen "to line" (see line (v.1)). Extended use, "covering or inner surface of anything," is from 1713. Meaning "action of providing with a lining" is from 1839.

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.

"give no indication of life, cease to function," by 1998, from the flat (adj.) line (n.) on an electrocardiogram or electroencephalogram when the patient is dead. Related: Flatlined; flatlining.

1859, "continuous conduit of pipes chiefly laid underground," from pipe (n.1) + line (n.). Figurative sense of "channel of communication" is from 1921; surfer slang meaning "hollow part of a large wave" is attested by 1963.

also re-line, "line (a coat, painting, etc.) again or anew, provide with fresh lining," by 1839, from re- "back, again" + line (v.1). Related: Relined; relining. 

1721, "to mark underneath or below with a line," from under + line (v.). Similar formation in Dutch onderlijnen. Related: Underlined; underlining. The noun is attested from 1888.

also base-line, "line upon which others depend," 1750, originally in surveying, from base (n.) + line (n.). In tennis, the end-line of the court (1872). The baseball diamond sense is from 1867. Baseline estimate was in use by 1983.

early 15c., liniament, "distinctive feature of the body, outline," from Latin lineamentum "contour, outline; a feature," literally "a line, stroke, mark," from lineare "to reduce to a straight line" (here apparently in an unrecorded sense "trace lines"), from linea "string, thread, line" (see line (n.)). Figurative sense of "a characteristic" is attested from 1630s.