In English and many other languages "bonus" means roughly "extra": You can earn bonus points as a frequent flyer or you can answer a bonus question in a game show. But the Latin adjective bonus means "good" (and some related things), but it does not seem to have the sense of "extra" or "additional". The online etymologies I could find say that the English word "bonus" comes from this Latin adjective but they do not explain how this happened.
When and how did the word "bonus" acquire this extra meaning? Did it happen in Latin or some other language?
If a reliable source in etymology states that we are unsure, that is a sufficient answer. I just think there should be some reason behind this, perhaps an individual event or example from which it spread.