Old English underwritan "write at the foot of; subscribe;" see under + write (v.). A loan-translation of Latin subscribere (see subscribe). Used literally at first; modern sense of "to accept the risk of insurance" (1620s) is from notion of signing a marine insurance policy. Meaning "to support by a guarantee of money" is recorded from 1890.
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Definitions of underwriter from WordNet
a banker who deals chiefly in underwriting new securities;
Synonyms: investment banker
an agent who sells insurance;
Synonyms: insurance broker / insurance agent / general agent