Even if you hide your thumb, you don't hide it by putting your hand in your pocket. It is said that the thumb is wrapped in the palm and the shape of the fist is made. The episode which seems to be a funny story that the finger seems to move reflexively, and the person who was riding the bicycle met the hearse, and it fell because the thumb of both hands was instinctively clenched is told.
Translations
Hide your thumb, not put your hand in your pocket and hide it. Wrap your thumb in the form of a fist clench. Because people who form habits will reflexively wrap their thumbs up, cyclists will subconsciously grasp their hands and fall down when they see the hearse. There have been similar jokes circulating.
The reason why the thumb is concealed is various in "Parents die prematurely" and "Parents are taken" and "Auspiciousness is bad" besides "Parents die prematurely" and "It is not possible to meet parents' deaths". In both cases, there is a tendency to understand that "hiding the thumb means hiding parents" by linking "thumb" and "parent" by the uneasy psychology that the disaster might fall on parents.
Why hide your thumb, the reason is that in addition to "parents will die early" and "will not be able to send their parents to the end", there are various reasons such as "parents will be taken away" and "unlucky". It is the psychology that parents may encounter accidents and other uneasiness at work, "hiding the thumb is the meaning of hiding the parents", you can associate the "thumb" and "parents" to understand.
The meaning of hiding the thumb is simply not cleared up, but it seems that the fact that it is done for funerals and hearse is intended to avoid the influence of necrulsity and impurities. You may think that it was to refuse a bad influence on the body voluntarily though it seems to have associated "Parents" from "Thumb" to worry about parents' lives such as "Because parents die prematurely".
You can't simply explain the meaning of hiding your thumb. Funerals and hearse drives are held out of an attempt to avoid the effects of the dead and filth. Because "parents will die early", I am worried about the life of my parents, which is associated with "thumbs up". In fact, I originally wanted to block the bad effects on my body.
In fact, this curse used to be performed in various situations. "When walking on the road at night, it is not made into a fox when the thumb is made inside and it holds it", "You may hold the thumb of both hands in to avoid the epidemic", and "When it encounters the fierce dog, the thumb of both hands may be made inside, the fist is gripped, and dog's eyes are gripped", etc.
In fact, this curse has appeared in many scenes. "When walking at night, if you clench your fists and hide your thumbs, you will not be deceived by the fox" "If you want to avoid infectious diseases, it is good to clench your fists with both hands" "When you encounter a fierce dog, it is good to clench your fists with both hands and stare at the glasses of the fierce dog" and so on.
Thus, hiding the thumb purposely for the purpose of preventing disaster implies that it is a place in the body where the thumb is easily targeted by the evil one if the view is changed. At the beginning of the Meiji era, when cholera became popular, rumors were flying that foxes would bring cholera, where the thumbs of the limbs were threaded. This indicates that the thumb was believed to be the entrance of the evil spirit symbolized by the fox.
In this way, the thumb is specially hidden in order to prevent disasters, and if you change the perspective and hide the thumb, it will give people a hint that it is easy to be targeted by evil things. In the early Meiji period, when these behaviors were popular, rumors such as attracting foxes were rife, so the act of wrapping a rope around the thumbs of the hands and feet occurred. These are the invading portals that believe that thumbs up is evil spirits.