The reason that you put toothpaste on your eyebrows is to help keep the eyebrows clean and tidy.
When applying toothpaste to your eyebrows you take a small dab of toothpaste and put it on your finger and then take the toothpaste and rub it into your eyebrows.
Then after you apply the toothpaste to your eyebrows with your finger and rub it in use a toothbrush and then use the toothbrush to smooth and shape the eyebrows.
If you want thicker eyebrows you should shave or trim the eyebrows which helps promote the growth of your eyebrows.
Vitamin E oil and eyebrow serum is also another good way to get your eyebrows to grow faster and thicker as well.
If you do use toothpaste on your eyebrows just be aware that some people may get irritation from the toothpaste due to sensitive skin.
So if you have sensitive skin you may not want to use toothpaste on your eyebrows often or at all.