to write under or at the foot of, especially under other written matter.
2.
to sign one's name, as to a document.
3.
to show agreement with or to support by or as if by signing one's name to, as a statement or decision.
4.
to bind oneself to contribute a sum of money to (an undertaking):
Wealthy music lovers underwrote the experimental concerts.
5.
to guarantee the sale of (a security issue to be offered to the public for subscription).
6.
Insurance.
to write one's name at the end of (a policy), thereby becoming liable in case of certain losses specified in the policy.
to insure.
to assume liability to the extent of (a specified sum) by way of insurance.
to select or rate (risks) for insurance.