Take agrimony and elm bark, root of vervain, root of willow, southernwood, burnt and pulverized linseed, [and] root of reed. Cook all these things with goat milk or water, and wash the area (having first shaved it)."
To my great surprise, this recipe does not seem harmful. (At least, not externally.) Several ingredients are recognizable herbs (vervain is often used for insomnia, for instance) and while others like southernwood should not be ingested, they are not particularly known for causing any external damage or discomfort. All the same, if you want to try this, use as much caution as you always should around new products-- and watch for allergies. As to whether it will make your hair thick... that I really don't know.
Source: The Trotula, 12th century