To support or guarantee something financially; to accept financial responsibility for a venture or to provide backing for an enterprise.
Originally meaning 'to write underneath', from the practice of signing one's name below a document to accept responsibility. In marine insurance, underwriters would literally write their names under the terms of a policy to show they accepted the risk, which extended to any form of financial backing.
Underwriters at Lloyd's of London literally wrote their names under insurance policies - the more names underneath, the more the risk was spread! Your signature 'under' the terms meant you were willing to pay if something went wrong, which is why we still say someone 'underwrites' a project when they provide financial backing.
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