flu n.

type of infectious disease, now known to be caused by a virus, usually occurring as an epidemic, with symptoms similar to a severe cold along with high fever and rapid prostration, 1743, borrowed (during an outbreak of the disease in Europe), from Italian influenza "influenza, epidemic," originally "visitation, influence (of the stars)," from Medieval Latin influentia in the astrological sense (see influence).

AN Article from Rome informs us that a Sort of Plague has broke out there, which destroys Abundance of their People, and they call it the Influenza. [The Gentleman's Magazine, April 1743]

Used in Italian for diseases at least since 1504 (as in influenza di febbre scarlattina "scarlet fever") on notion of astral, occult, or atmospheric influence. The 1743 outbreak began in Italy. Often applied since mid-19c. to severe colds. For the sense development, compare Latin sideratio "blast, blight, palsy," from siderari "to be planet-struck, afflicted as if by an evil star."

1610s, "small piece of paper with writing on it, a written slip," apparently a corruption of script (n.). In the commercial use, "a certificate of a right to receive something" (especially a stock share), 1762, in this sense probably shortened from (sub)scrip(tion) receipt (see subscription). Originally "receipt for a portion of a loan subscribed;" the meaning "certificate issued as currency" is recorded by 1790. In U.S. history, "fractional paper money" (by 1889).