Moral utility is a somewhat, but not entirely, outdated concept in patent law—that an invention designed for an immoral purpose should not be patentable.
The man behind moral utility is widely believed to be Supreme Court Justice Joseph Story, who came up with the concept while riding circuit in Massachusetts in 1817. He said that utility could not be found in inventions that "poison people, or promote debauchery, or facilitate private assassination."
Moral utility was used to invalidate many gambling machine patents in the 19th and early 20th centuries. A number of other patents were invalidated simply because they could be used for gambling—inventions like coin return devices and horse racing games. However, by the 1970s, gambling machines were routinely receiving patents despite the moral utility arguments.
For many years, moral utility was also used to deny patents that were fraudulent in some way. One well-known early example was a patent for seamless stockings with fake seams: at the time (the 1920s), stockings with seams were widely viewed to be higher-quality than seamless stockings, so the fake seams were seen as an attempt to defraud consumers. This line of jurisprudence ended in 1999, when the Federal Circuit upheld a patent for the Juicy Whip machine. The Juicy Whip is a lemonade dispenser which circulates an inert yellow liquid inside a visible tank while dispensing actual lemonade from a hidden tank below the counter: the Federal Circuit said that deceptiveness should not affect the utility of an invention.
In the old days, patents were also denied to medicines that had questionable safety. Today, the Patent Office leaves such issues to the judgment of the FDA, and does not deny drug patents on the basis that the drug is unsafe.
An obvious problem with the doctrine is that the morality of an invention is in the eye of the beholder. And besides that, many inventions have unintended consequences. The television and the automobile have had many negative effects on American society, but at the time they were introduced, who would have foreseen such effects? These are arguments that keep moral utility out of most patent cases today.