genus of plants native to Mexico and Central America, 1804, named 1791 by Spanish botanist Antonio José Cavanilles for Anders Dahl (1751-1789), Swedish botanist and pupil of Linnaeus, who discovered it for science in Mexico in 1788.
The likelihood that a true blue variety of the flower never could be cultivated was first proposed by French-Swiss botanist Augustin Pyramus de Candolle, and noted in English by 1835; hence blue dahlia, figurative expression for "something impossible or unattainable" (1843).