It is thought that the word Bully comes from the Middle High German ‘brother’ which then led to the Dutch word ‘boel’ which translates to ‘lover’. It is for this reason that in the 16th century you were much more likely to hear someone refer to their girlfriend as a bully then the guy who tripped them up in the playground.
The development of the meaning, from a lover to what we now think of as a bully, is a gradual one. From ‘fine fellow’ to ‘blusterer’ which meant someone full of empty threats it eventually came to mean ‘harasser of the week’. However it wasn’t until the 20th century that ‘bullying’ began to have a real definition as it does today. In fact it wasn’t until 1978 that the Swedish academic Dan Olweus, who described the term in his book Aggression in the Schools, that a more concrete definition was formed. According to him bullying includes repeated, deliberate verbal or physical abuse from someone with more power than their target.
Of course this definition is up for debate and the parameters of what is considered bullying are constantly changing. Dr Olweus would have never heard of the internet and as cyberbullying has become such a concern more and more definitions are emerging.