What did the word bully come from?

The origin is not clear, particularly as the Word has changed its meaning over the years. There was a Dutch Word from the Middle Ages also used in English 'boel' that translated as 'sweetheart'. Over the years the Word deteriorated from 'fine fellow' to 'blusterer' to 'harasser of the weak'. However, as a slang Word as in 'bully for you' it retains some of its original meaning by referring to an individual as a 'fine fellow'