To assume financial responsibility for; agree to pay the costs of: underwrite a theatrical production. To supply with funding, especially as a sponsor: underwrite a public television show. To sign (an insurance policy) so as to assume liability in case of specified losses. To insure. To insure against losses totaling (a given amount). To guarantee the purchase of (a full issue of stocks or bonds). To agree to buy the unsold part of (stock not yet sold publicly) at a fixed time and price. To write under or at the end of something. To subscribe to, especially to sign or endorse (a document). To support or agree to (a decision, for example).