Douglas Harper's Etymology Dictionary
1894, from French phrase en plein air, literally "in the open air." The style developed among French impressionists c.1870.
Usage examples of "plein-air".
Prints of plein-air landscapes and still lifes favoring fruit and fish hung on the walls, along with a bronze Zodiac wheel and an old Christmas wreath.
Georgia O’Keeffe prints hung on the walls, along with some muddy-looking California plein-air oils.
On the walls was a scattering of muddy California plein-air landscapes—the kind of stuff the galleries in Carmel had been trying to palm off as masterpieces for years.