"Shonin" is a term of respect for a high priest in Buddhism. Originally it referred to a person who was knowledgeable and virtuous in "Mahaprajnaparamita-sutra," as a means to show respect.
In Japan, it was a symbol of Hokkyo-shonin (a title of honor given to Buddhist sculptors, painters, poets, etc.), which was one of the ranks for monks in 864; later, however, the monks who traveled around and put their efforts into teaching people were referred to as Shonin (上人) or Shonin (聖人) by ordinary people. Kuya is said to have been one of the first monks who was called Shonin.
Subsequent to the Muromachi period, there was a custom of calling monks 'Shonin,' specifically those who had received rinji (the Emperor's command) of the Shonin title.
Incidentally, some religious schools they have certain titles that are used for Shonin go. In the Jodo (Pure Land) sect, the Japanese character '誉' was used for the bottom character of the name, and in Jodo shin shu (the True Pure Land sect of Buddhism) the Japanese character '如' was used for the bottom character of the name (however, the Shonin title was used for the chief priest of Hongan-ji Temple, whether he received rinji or not), while in the Ji sect the character '阿' was used for the bottom character of the name, and in the Nichiren sect the character '日' was used for the top part of the name.