A "Subprime borrower" Is somebody who has a poor credit history or some other indication that would suggest that they might not be able to repay the mortgage-- they might default.

次级借款人"指的是,那些信用记录较差的借款人,或在其他方面显示,他们很可能无力偿还贷款,也就是说他们很可能违约

耶鲁公开课 - 金融市场课程节选