English
adult baby
From adult + baby.
The story
Popularized 1980s, largely replacing infantilist outside psychology.
Today it means
noun An adult person who enjoys role-playing a regression to an infant-like state and acting like a baby or toddler, as through wearing diapers, engaging in babylike activities, babbling, etc., generally for sexual gratification.
Language periods are conventional ranges, not dates for this word.
Etymograph / adult baby