WordNet (r) 3.0 (2006):

bad debt
    n 1: a debt that is unlikely to be repaid

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "bad debt":
   accounts payable, accounts receivable, amount due, bad debts, bill,
   bills, borrowing, charges, chits, debt, default, defection,
   delinquence, delinquency, dishonor, dishonoring, due, dues,
   financial commitment, floating debt, funded debt, indebtedness,
   indebtment, liability, maturity, national debt,
   nondischarge of debts, nonpayment, nonremittal, obligation,
   outstanding debt, pledge, protest, protested bill, public debt,
   repudiation, score, uncollectible, uncollectibles,
   unfulfilled pledge