means The small amount of water or other liquid present in the air, in a surface, or within a substance — enough to feel damp but not enough to be wet.

from From Latin 'mustum,' the fresh unfermented juice pressed from grapes, which gave Latin 'mustus' (fresh, new). This passed into Old French as 'moiste' (damp, fresh) and into English as 'moist,' with 'moisture' following close behind. So the grape-and-raisin joke is no accident: the word was literally born wet, in the juice of crushed grapes.