The claim against a person who lent a car to someone they knew, or should have known, was unfit to drive it - the owner's own negligence, separate from the driver's.
Negligent entrustment is the owner's own wrong, not a rule that makes owners answer for drivers automatically. The claim is that the person who supplied the vehicle knew or should have known that the person they gave it to was likely to drive it dangerously - because they were intoxicated, unlicensed, too inexperienced, medically unfit, or had a history of reckless driving the owner was aware of - and that the foreseeable harm followed. The injured person must prove the driver's negligence and, separately, the owner's knowledge; a careful lender who had no reason to suspect anything is not liable under this doctrine even if the borrower causes a crash.
The same theory reaches employers who hand company vehicles to employees with disqualifying driving records, parents who let a child with a known pattern of crashes keep using the family car, and dealers or others who supply a vehicle to someone visibly impaired. It is also the reason a rental company or a lender that runs a business of supplying vehicles is still exposed even where a federal statute protects it from liability based on ownership alone: the Graves Amendment bars claims against rental and leasing companies that rest only on the fact of ownership, but expressly leaves their own negligence, including negligent entrustment, untouched.
Negligent entrustment matters in practice for two reasons. It reaches an owner's insurance and assets when the driver has little of either, and it opens the owner's knowledge to discovery - what they knew about the driver's record, licence status and habits - which is often the most contested evidence in the case.
An injured person whose claim against the driver is limited by a thin policy should ask a lawyer early whether the owner is a separate defendant, because the answer turns on evidence - licence records, prior incidents, what the owner had been told - that has to be requested before it disappears. An owner who is being sued on this theory needs advice on the same evidence from the other side, and on whether their own insurer is defending the claim.
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