Why don't old people at home let their children cut their toenails and fingernails at night?

My mother told me that if I cut my nails at night, I would become a thief. When you cut your toenails, you will stumble when you walk. Although I know it's not true, I don't have the habit of cutting my nails at night? And some fortune tellers. Actually, I was scared, too. I don't eat, drink or sleep. I always hear what I shouldn't hear in the middle of the night. Actually, I am too weak. When I get better, there won't be such a "thing"?