A person from a tale says:「 へそをかくせー」 because they see the God of Thunder (雷様) approaching. What does the sentence mean?
かくせー： imperative of 隠す｛かくす｝or to hide.
Literally translated, this means:
Cover your navel!
Sounds kindof crazy, but there's a cultural meaning behind it. Frequently, parents will say something to the effect of "The god of thunder and lightning will take your belly button, so cover it up." As far as I can tell, it's an old wives' tale that parents tell their children.
I found the following explanation as to where this saying came from, but it's all in Japanese. I found it on this webpage.
I'll copy one of the explanations here.
My translation of this is not perfect, but it is sufficient for understanding what was written above.
In addition to the discussion from earlier, there are yet other reasons why someone would cover their navel. When you move to cover your navel, what kind of posture do you assume?
When you stretch your back I doubt you or anyone else would put your hands on your stomach, right? I think that most people round their backs and become smaller when they do so.
It is said that this is to protect your body from the God of Thunder and Lightning.
It is the nature of lightning to strike the high ground. It strikes tall trees, towers, etc. To protect your navel, you stoop over. By covering your body you naturally protected yourself from lightning strikes.
Now lets talk about the time people usually wore kimonos. People would walk around with their wallets in their obi, and it appears that some of those people were struck by lightning. The navel on those people struck by lightning was charred black, making it look like their belly button had been taken.
Because the skin around the navel is thin, it can completely split open or… you can get shocked from electricity entering your body.
Because lightning strikes are the cause of this coincidence, the God of Lightning has taken your belly button! Or at least, that’s why people started to say he likes belly buttons.
So you see, covering the navel is actually done out of necessity to properly protect oneself from lightning.
Next time you go outside during a thunder storm, watch out. You might lose your belly button!