A more thorough explanation:

ad mordendum assuetus

Ad mordendum assuetus is a legal term that means "accustomed to bite." It was often used in cases where a dog had caused harm to a person or another animal.

If a dog had bitten someone before, it could be considered ad mordendum assuetus. For example, if a dog had bitten a mail carrier, and then later bit a child, the dog could be considered ad mordendum assuetus. This means that the dog had a history of biting and was therefore more likely to do it again.