A more thorough explanation:

Definition: Physical weakness caused by age or disease; especially, in insurance law, an applicant's ill health that is poor enough to deter an insurance company from insuring the applicant.

Example: The old man's infirmity made it difficult for him to walk without assistance.

Explanation: The example illustrates how physical weakness caused by age can be referred to as infirmity. The old man's inability to walk without assistance is a clear indication of his infirmity.

Another example could be an insurance company denying coverage to an applicant due to their infirmity, which is their poor health caused by a disease or condition.