mid-15c., "pertaining to the sun," from Latin solaris "of the sun," from sol "sun" (from PIE root *sawel- "the sun"). Meaning "living room on an upper story" is from Old English, from Latin solarium (see solarium). Old English had sunlic "solar."
Astrological sense from 1620s. Meaning "operated by means of the sun" is from 1740; solar power is attested from 1915, solar cell from 1955, solar panel from 1964. Solar system is attested from c. 1704; solar wind is from 1958. Solar plexus (1771) "complex of nerves in the pit of the stomach," apparently so called from its central position in the body (see plexus).