The phrase "butter someone up" means to flatter someone excessively in order to gain their favor or to achieve a desired outcome. The origin of this phrase can be traced back to ancient practices and cultural traditions. 

Here’s a detailed look at its origin: 

One commonly cited origin of the phrase "butter someone up" comes from ancient India. It was a customary practice to throw balls of clarified butter (ghee) at the statues of gods in order to seek favor and blessings. 

This act of literally "buttering up" the deities was a way to gain their goodwill and favor. In some Buddhist traditions, devotees would offer butter sculptures to the gods as a sign of respect and to gain favor. These intricate butter offerings were made during religious ceremonies and festivals, and they were believed to please the deities and ensure their blessings. 

Over time, the literal act of "buttering" to gain favor evolved into a metaphorical expression. The phrase began to be used to describe the act of flattery or excessive praise to win someone over or achieve a particular goal. By the 19th century, the phrase was widely used in English-speaking countries to describe the act of flattering someone for personal gain.