[Latin, He undertook or he promised.] A promise by which someone assumes or undertakes an obligation to another person. The promise may be oral or in writing, but it is not under seal. It is express when the person making the promise puts it into distinct and specific language, but it may also be implied because the law sometimes imposes obligations based on the conduct of the parties or the circumstances of their dealings.
Assumpsit was one of the common-law forms of action. It determined the right to sue and the relief available for someone who claimed that a contract had been breached.
When the common law was developing in England, there was no legal remedy for the breach of a contract. ranulf glanvill, a famous legal scholar, wrote just before the year 1200 that "[i]t is not the custom of the court of the lord king to protect private agreements, nor does it concern itself with such contracts as can be considered private agreements." Ordinary lawsuits could be heard in local courts, but the king was primarily interested in royal rights and the disputes of his noblemen. As commerce began to develop, the king's courts did allow two forms of action for breach of contract—the actions of covenant and debt. Covenant could be maintained only if the agreement had been made in writing and under seal and only if the action of debt was not available. One could sue on the debt only if the obligations in the contract had been fully performed and the breach was no more than a failure to pay a specific sum of money.
Finally, in 1370, a plaintiff sought to sue a defendant who had undertaken to cure the plaintiff's horse but treated it so negligently that the horse died, and the action was allowed. In 1375, another man was permitted to sue a surgeon who had maimed him while trying to cure him. These cases showed a new willingness to permit a lawsuit for monetary damages arising directly from the failure to live up to an agreement. For the next hundred years the courts began to allow lawsuits for badly performed obligations but not for a complete failure to perform what was required by contract. Unexpectedly, this restriction was abandoned also, and a new form of action was recognized by the courts, an action in special assumpsit for breach of an express agreement.
Special assumpsit gave a new legal right to parties who could not sue on a debt. Gradually, it became possible to sue in assumpsit if the defendant owed a debt and then violated a fresh promise to pay it. This action came to be known as indebitatus assumpsit, which means "being indebted, he promised."
As time passed, courts were willing to assume that the fresh promise had been made and to impose obligations as if it had. This allowed lawsuits for a whole range of contract breaches, not just those recognized by an action on the debt or in special assumpsit. If the plaintiff could claim that services had been performed or goods had been delivered to the defendant, then the law would assume that the defendant had promised to pay for them. Any failure to do so gave the plaintiff the right to sue in assumpsit. This development allowed such a wide range of lawsuits based on promises to private parties that it came to be known as general assumpsit.
Eventually, the right to sue was extended even to situations where the defendant had no intention to pay but it was only fair that he or she be made to do so. This form was called assumpsit on quantum meruit. Special assumpsit, general assumpsit (or indebitatus assumpsit), and quantum meruit are all ex contractu, arising out of a contract. Their development is the foundation of our modern law of contracts.