I have looked everywhere and have found no clues. The term originated in France, so it is unlikely that it has anything to do with All Souls College. It first appears in print in 1900. It is assumed that buttock is a diminutive of butt and not the other way around. Butt in the anatomical sense is first recorded in the mid-15th century.
Bottle is related.
So if you get thee to a buttery, it does not have to be a fattening experience. Nowadays it means the opposite, as virtual reality and virtual money are neither real nor money. Virtue , the English noun which derives ultimately from virtus , came from Old French vertu in the 13th century. Virtual is almost as old, you may be surprised to learn, entering English in the 14th century. I was recently required to complete a form about my employment which asked who I reported to and who my associates were.