“To butter someone up” is to flatter or be nice to someone in order to get something from them. “Flatter” is a synonym, since it means to seek a favor by excessive praise.
There are two possible origins of this idiom. Some people believe that it comes from actually spreading smooth, creamy butter on a slice of bread which can be compared to spreading nice words on a person. Others believe that it originated in ancient India where they used to throw balls of butter at statues of gods to ask for a favor.
One of the early examples of the figurative meaning comes from the Saturday Review of July 5, 1884: “The Lord Chief Justice of England made a tour through America and generously buttered the natives.” (From Dictionary of Cliches by James Rogers, 1985)
History tells us that in order to seek divine favour, Hindu worshippers would throw balls of ghee (clarified butter used as the foundation of Indian cooking) at statues of their deities. By “buttering up” the gods, it was hoped that the worshippers would be rewarded with peace and good harvests.
We don’t know when this began, but the guides of the Meenakshi Amman Temple in Madurai, Tamil Nadu, built circa 1600s BCE, speak of the ancient custom.
A similar practice can be seen in the Tibetan New Year celebrations, where sculptures made out of colored butter would be displayed as gifts to the heavens. This ritual can be traced back to the Tang Dynasty (CE 618-907).
I’m not a fan of butter, but if honey could be used, I’d be willing to stand in as one of the gods.