Looking at Etymonline and Dictionary.com only reveals that it was slang from 1759. Why did bumper come to mean unusually abundant, and why is it always paired up with the word crop?
Learn English – the etymology of the adjective “bumper”
adjectivesetymology
Best Answer
The Oxford English Dictionary provides an enlightening quotation under the second meaning it gives, which is "anything unusually large or abundant." The quote is from 1759 and came from The Gentleman's Magazine:
It then has quotes which uses bumper in various contexts, as for a large sum of money, a high score in a game, a very full brook, etc. Only one of these uses the now predominant form of "bumper crop".
The OED does not offer a definitive etymology, although it does suggest it may have come from the verb bump. It does seem plausible that in casual speech an unusually large object might have come to be called a "bumper" from the impact it might make when deposited onto a surface.