When this saying was invented, people really had to keep busy or else they’d die. It comes from the Bible. It was written down by people who had enough slaves to have enough time left over to write. If you were a slave and didn’t keep busy, your master could kill you. The good thing about having a master was that masters had herds of goats and that meant you didn’t have to worry about starving if you didn’t find an animal to kill within two weeks.
If you didn’t have a master, you had to stay even busier. We’re so arrogant about our technology that we think we invented multi-tasking. But I’d be willing to bet that nobody reading this has ever had to build a fire to cook breakfast before making thread on a spinning wheel from cotton you picked yesterday to weave enough cloth on a loom to make a blanket before giving birth this afternoon.
It wasn’t just women, either. Men had to make arrows and spears while they were hunting. People with bad aim can only carry so many, right? They didn’t have eyeglasses, then, either. If a man didn’t hunt down an animal, kill it, butcher it and get it home in time for dinner, he’d also have to go hunt for more firewood.
Don’t think all the king’s men didn’t take advantage of women left alone while their men were out hunting, either. Promising to stop raping their wives was one of the strategies they used for getting men to join their gang and serve the king. People without masters hardly ever had time to sit around plotting how to get far enough away to be free of them or overthrow the king to get some land and slaves of their own.
That was the whole point of King James putting that saying in the book when he edited it. His version promised people who worked hard a place in heaven while threatening those that didn’t with an eternal hell even worse than the one they were already in. As long as people stay super busy, kings are safe.