The Adept NPC class is a good interpretation that WotC did on the "Hedge Wizard" Archetype. I know the term has been around AT LEAST since the early 1990's, having seen it in fan mag's and in some very old internet RPG documents (stuff that was passed around via ftp back in "the olden days." It's origins, however, I can't pinpoint.
Simply put, it's a charm-weaver; a rural mage; the witch that sells 10 gp love potions, and works cheap magics for those who can afford them. Old 1st edition AD&D used to have NPC classes as well, Gary just didn't call them as such; they were unleveled, and they were sages, shamans, witch doctors, etc. That had powers that never broke 2nd or maybe 3rd spell level in power, IIRC.