1650s, "not properly attentive" (of persons), a back-formation from inadvertence. Meaning "unconscious, unintentional" (of actions) is from 1724.
Entries linking to inadvertent
inadvertence n.
"carelessness, negligence, inattention," mid-15c., from Old French inadvertance "thoughtlessness, heedlessness" (14c.), from Scholastic Latin inadvertentia, from in- "not" (see in- (1)) + advertentia, from Latin advertere "to direct one's attention to," literally "to turn toward" (see advertise).
inadvertently adv.
1670s, from inadvertent + -ly (2). "Inattentively, carelessly," hence "unintentionally."