The elixir of legend is a liquid you drink to live forever. The word started out dry.
Etymonline traces elixir to Arabic al-iksir, and the Arabic word probably to late Greek xerion, a powder for drying wounds. Xerion comes from xeros, "dry." The al at the front is the Arabic article, "the."
In Arabic and then Latin alchemy, al-iksir named the philosopher's stone. Etymonline says the Medieval Latin elixir meant exactly that. Alchemists believed it could turn base metals into gold, cure disease and prolong life. English picked up the word in the Middle Ages with that meaning. Etymonline dates it to the mid-13th century, while Dictionary.com gives 1350 to 1400.
The idea of a life-giving substance is much older than the word. Wikipedia's article on the elixir of life gives examples from several cultures. The Qin emperor Qin Shi Huang sent the alchemist Xu Fu east with 500 young men and 500 young women to find it on Penglai Mountain. Xu Fu came back empty-handed. A second voyage, with 3,000 young people, never returned.
Many of these remedies were dangerous. An alchemical text attributed to the Chinese physician Sun Simiao, who lived roughly from 581 to 682, describes elixirs containing mercury, sulphur and arsenates. In Hindu tradition, the gods retrieve amrita by churning an ocean of milk.
In English the word slowly came down to earth. By the 1590s an elixir could be a strong tonic, according to Etymonline. In pharmacy it came to mean a tincture with more than one base. From the 1630s it was also used for quack medicines.
The dry root is still at work in modern English. Xero- means "dry," and Etymonline dates xerography, copying without liquid developers, to 1948. Its trademark, Xerox, followed in 1952. A photocopy and an elixir share a Greek word for dry.
Like alchemy, elixir reached Europe from Arabic with the article al- fused to the front.