[thombat]

Normally used to mean an activity in the fresh air, generally with pleasant overtones. So dining "Al fresco" might be a meal on a terrace, or a picnic in a park. Flying al fresco could be a jolly ride in a biplane or a hot air balloon, glass of bubbly in hand, not so much the scream of air through your shattered canopy as you guide your crippled jet to the ground.

That student plonk was aiming at debonair status; the favourite of my student days was a cooking wine with the grand title of Red Ridge Claret a little undermined by arriving in a 3L cardboard box, which stacked nicely on the mantelpiece as trophies of just how sophisticated we were.

[billfruit]

With regards to painting it seems to mean work done on top of fresh plaster.

However the term "en plein air" means works executed in the outdoors.