For example, some places say "four don't borrow" are medicine pots, pots and axes, some places say "cribs, pig-killing knives, crutches and medicine pots", and some places say houses, personal belongings, newly married couples' things, cars and so on, but what people don't borrow is something that everyone thinks is taboo.
Generally speaking, "four don't borrow" means cribs, medicine cans, sharp tools and crutches. I don't want to borrow a crib, because people in the old society thought that this baby's cradle represented the prosperity of this family. If I lend it out, I'm afraid it will be sparsely populated in the future, which will be very bad for my host family.
In ancient times, people used traditional Chinese medicine to treat diseases, which needed to be boiled in a medicine pot. Therefore, this medicine pot represents pain. If you lend the medicine can to others, and you can't handle it properly, it will bring a painful disaster to others, and then bring it back when others return it.
Sharp tools are sharp things, but also weapons, such as knives, for fear that others will take them out to do bad things. The other is an axe, which is homophonic with the word Fu, so it is not borrowed. Although the workmanship is simple, it plays a very important role in people's minds.
Crutches are things that disabled people or sick old people will use, that is to say, when they lend them out and return them, they will also bring diseases and disasters to their host families.
Although this is an old saying, every place has different customs, but their basic meaning is almost always the desire not to do harm, to avoid diseases and disasters, and to hope that one's family can be happy. Although there is no scientific basis, it is a custom. Some things are lent to others not for their own good, but also for others' good.