The Collaborative International Dictionary of English v.0.48:

Uncollectible \Uncollectible\
   See collectible.

WordNet (r) 3.0 (2006):

uncollectible
    adj 1: not capable of being collected; "a bad (or uncollectible)
           debt" [syn: bad, uncollectible]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "uncollectible":
   accounts payable, accounts receivable, amount due, bad debt,
   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, uncollectibles, unfulfilled pledge