Debt overhang is the condition of an organization (for example, a business, government, or family) that has existing debt so great that it cannot easily borrow more money, even when that new borrowing is actually a good investment that would more than pay for itself.
This problem emerges, for example, if a company has a new investment project with positive net present value (NPV), but cannot capture the investment opportunity due to an existing debt position, i.e., the face value of the existing debt is bigger than the expected payoff. Hence, the equity holders will be reluctant to invest in such a project because most of the benefits will be reaped by the debt holders. In addition, debt holders will not finance the firm if the company cannot convince the debt holders that the project will not fail.
The situation emerges if existing debtholders of a company can be expected to lay claim to (part of) the profits of the new project, and this renders the NPV of the project (when undertaken by this company) negative.