Dictionary.net

He owes and detains. Words anciently used in the original writ, (and now, in English, in the plaintiff's declaration.) in an action of debt, where it was brought by one of the original contracting parties who personally gave the credit, against the other who personally incurred the debt, or against his heirs, if they were bound to the payment; as by the obligee against the obligor, by the landlord against the tenant, etc. The declaration, in such cases, states that the defendant "oices to," as well as "detains from," the plaintiff the debt or thing in question; and hence the action is said to be "in the debet ct detinet." Where the declaration merely states that the defendant detains the debt, (as in actions by and against an executor for a debt due to or from the testator,) the action is said to be "in the detinet" alone. Fitzh. Nat. Brev. 119, G.; 3 Bl. Comm. 155.