A debt that is unlikely to be repaid WordNet 3.0 & Open English WordNet

Bad debt, occasionally called uncollectible accounts expense, is a monetary amount owed to a creditor that is unlikely to be paid and for which the creditor is not willing to take action to collect for various reasons, often due to the debtor not having the money to pay, for example due to a company going into liquidation or insolvency. Wikipedia

Financial asset whose value has fallen significantly and for which there is no longer a functioning market Wikidata

A debt which cannot be recovered from the debtor, either because the debtor lacks the money to pay or because the debtor cannot be found and/or forced to pay. Wiktionary

Unrecoverable debt. Wiktionary (translation)