The tiny, innocent buttercup at some point acquired the reputation of causing lunacy and was called the "crazy weed". To hold the flower next to your neck on a night when the moon is full, or simply to smell the flower, were said to drive you insane.
Since the juice of the buttercup quickly raises a blister on the skin, it was often used by beggars in an attempt to create more sympathy.
A story of how the buttercup got the name Ranunculus was that this was the name of a Libyan boy who could sing very beautifully. He always wore green and gold silk, and he was constantly singing. One day while he was singing in the woods, the wood nymphs heard him. To get some peace and quiet, they turned him into a green and gold flower, which we call the buttercup today.
An old custom is to hold the blossom under your chin and if your chin shines yellow, you love butter.