Green’s Dictionary of Slang

(US) used of a temporary replacement for an item being repaired; of anything on loan/hire; also as n.

1981		J. Ellroy Brown’s Requiem 236: I left that night, leaving my loaner car in the lot at L.A.X.
2007		‘Wife Watcher’ ‘Commuter Train’ in asstr.org 🌐 Our car was at the garage for repair and they didn’t have a spare ‘loaner’.
2009		D. Lehr Fence 98: Tiny had parked a 1994 gold Lexus, a ‘loaner’ from the [car] dealer.