plexus (n.)

mid-15c., "of, pertaining to, or determined by the sun," from Latin solaris "of the sun," from sol "sun" (from PIE root *sawel- "the sun"). Meaning "living room on an upper story" (also sollar) is from Old English, from Latin solarium (see solarium). For "of or proceeding from the sun," the earlier word was Old English sunlic, Middle English sonneli. Solific (1550s) also has been used.

The meaning "operated by means of the sun or its heat" is from 1740; solar power is attested from 1915, solar cell as a photovoltaic device from 1955, solar panel, designed to absorb the sun's rays, is from 1964. The astronomical solar system "sun and the bodies revolving round or dependent on it" is attested from c. 1704; solar wind is so called 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).

1849 in yoga sense of "spiritual centers of power in the human body," from Sanskrit cakra "circle, wheel," from PIE root *kwel- (1) "revolve, move round."

The Tantric theory on which the well known yoga called Shat-chakra-bheda is founded, supposes the existence of six main internal organs, called Chakras, or Padmas, all having a general resemblance to that famous flower the lotus. These are placed one above the other and connected by three imaginary chains, the emblems of the Ganges, the Jumna, and the Saraswati. ["Physical Errors of Hinduism," The Calcutta Review, 1849.]

Connection of the chakra with the anatomical plexus is by 1897. As a small projectile wheel or disc, typically appearing in Hindu religious art, also attested 1849 as a Sanskrit word in English.