1839, flue, shortening of influenza. Spelling flu attested from 1893. The abstraction of the middle syllable is an uncommon method of shortening words in English; Weekley compares tec for detective, scrip for subscription.
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.
"certificate of a right to receive something" (especially a stock share), 1762, probably shortened from (sub)scrip(tion) receipt. Originally "receipt for a portion of a loan subscribed," meaning "certificate issued as currency" first recorded 1790.
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Harper, D. (n.d.). Etymology of flu. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/flu