When and how did "bonus" get to mean "extra"?

Question

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.

Answer 1

I think this one is pretty simple. If you purchase something, and you get what you bought plus a little extra, isn't that a good thing?

As for the specific history, according to Etymonline and Oxford English Dictionary, it originated in the Stock Exchange as slang:

(From OED) An ignorant or jocular application of Latin bonus ‘good (man)’, probably intended to signify a boon, ‘a good thing’ (bonum). Probably originally Stock Exchange slang

You might notice that in the UK, it can also mean "an extra dividend or issue paid to the shareholders of a company." It likely, on account of the bad grammar, was jocular at first, too, and then became normalized—same thing with the word OK. The OED lists this as their earliest attestation:

1785 C. Macklin Man of World iii. 37 Got my snack of the cloathing..the contracts, the lottery teeckets, and aw the poleetical bonusses.

The spelling here doesn't indicate the lack of a formalized spelling system, but rather imitates pretentious yet uneducated speech, since it's found in the mouth of one Sir Pertinax Macsycophant. (There may be a gibe at Scottish pronunciation in there, too, but I'm not expert in this literature.)