"residing on the premises," 1950, from live (v.) + in. Lived-in "inhabited, occupied" is first recorded 1873.
Sharing one's domicile: Coe's former live-in girlfriend/ J Edgar's ''longtime live-in lover'' (1955+)
A housekeeper or caregiver who lives in one's home: After they had their second child, they hired a live-in (1955+)