For some reason, English has tended to borrow words related to death from French. Not directly related to somebody dying, for the most part — just some possibly unexpected connection to death in general.
The most common example is of course “mortgage”, which is borrowed from French where it means “dead” (mort) “pledge” (gage). Sometimes this is defined as “pay until you die,” but that wasn’t the original meaning. It’s called a “dead pledge” or a “dead deal” because it’s the deal itself that dies when either the debt is fulfilled or the payment fails.
Another French word with a similar idea of “death” is “mortmain.” This is very occasionally used in English, but you’d be pretty lucky to run across it nowadays. It was much more common in medieval times, where it was taken up in England from French property laws. In medieval France, the leaders of churches were usually the ones in possession of both land and money. They didn’t want these things to become state property when they died (which was the way it worked in France). French property law had a way around that, though; the church itself was considered an individual. As an institution it couldn’t die, so the property of church officials was said to be controlled by the “mortmain” of the church. “Mortmain” means “dead” (mort) “hand” (main). Yeah, even though the institution couldn’t die, it was a dead hand. Who says words have to make any sense?
“Mortmain” was used in medieval England in the same way (apparently English churches also qualified as individuals). But the word, for some reason, was gradually replaced by its English translation: “dead hand.” “Dead hand” is still in use today; something can be “held by the dead hand of” an entity or system, and nowadays that means the thing being held is prevented from further progress. For example, “true equality is oppressed by the dead hand of racism.” Notice that in this case the implication is that the hand isn’t dead at all, but active. And, of course, an “ism” can’t die any more than an institution can.
“Dead hand” is also used as an adjective in “dead-handed,” which means simply oppressive. A teenager (at least an unusually well-read one) might complain about the dead-handed authority of their parents. A teenager of at least a century ago would be even more likely to mention it.
And in (another) unexpected departure, “dead hand” is also a slang expression for expertise — you might hear someone described as a “dead hand” at shooting foul shots in basketball. “Dead hand” in this sense is much more common in the UK than in North America, where our attention is too consumed by paying mortgages to worry about other kinds of dead hands.
