Imaginary
late Middle English: from Latin imaginarius, from imago, imagin- ‘image’.
wiktionary
From Middle French imaginaire, from Latin imāginārius(“relating to images, fancied”), from imāgō.
The mathematical sense derives from René Descartes's use (of the French imaginaire) in 1637, La Geometrie, to ridicule the notion of regarding non-real roots of polynomials as numbers. [1] Although Descartes' usage was derogatory, the designation stuck even after the concept gained acceptance in the 18th century.
etymonline
imaginary (adj.)
"not real, existing only in fancy," late 14c., ymaginaire, from imagine + -ary; or else from Late Latin imaginarius "seeming, fancied," also literal, "pertaining to an image," from Latin imaginari "picture to oneself." Imaginary friend (one who does not exist) attested by 1789.
