Suppose your business offers its customers payment terms such as Net 30 and Net 15, you will ultimately bump into a customer who will make excuses to pay you off or possibly he could not pay you in real at all. When such situations keep piling up, you call it bad debt expenses. 

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Debt bad are items or goods sold on credit to a customer and payment has not been made over a long period of time or years. That unpaid amount becomes  a burden or an expense to you the debtor. Therefore such an amount is term bad hence BAD DEBT and it is record as expenses in the Trading Profit and Loss Account.