1Originally in religious or legal use: a son or daughter who has reached the age of discretion or majority. Subsequently: a son or daughter who has reached adulthood.
(originally and chiefly US). An adult who grew up in a dysfunctional family, especially in the care of a parent or guardian with an addiction (frequently alcoholism).
3An adult who retains the qualities or characteristics typical of children; especially one who is not fully developed emotionally or socially.
Mid 17th century; earliest use found in Samuel Fisher (bap. 1604, d. 1665), Quaker preacher and writer. From adult + child.