From inn + mate, or from in- + mate.

inmate (plural inmates)

1. A person confined to an institution such as a prison (as a convict) or hospital (as a patient).
2. A person who shares a residence (such as a hotel guest, a lodger, or a student living on campus), or other place.
   • 1852 July, Herman Melville, “Book XVI. First Night of Their Arrival in the City.”, in Pierre: Or, The Ambiguities, New York, N.Y.: Harper & Brothers, […], →OCLC ↗, section I, pages 312–313 ↗:

     [T]he inmates of the coach, by numerous hard, painful joltings, and ponderous, dragging trundlings, are suddenly made sensible of some great change in the character of the road.

• French: détenu, détenue, codétenu, codétenue
• German: Insasse, Insassin
• Italian: detenuto, internato, prigioniero
• Portuguese: interno
• Russian: заключённый
• Spanish: internado, interno, preso, recluso, presidiario, reo, convicto, detenido

• French: résident, résidente, pensionnaire
• German: Bewohner, Bewohnerin
• Italian: interno, residente
• Russian: жи́тель
• Spanish: residente