Wordorigins.org

To the modern ear this phrase sounds odd. How did bumper become associated with agriculture?

The original bumper was a large cup, filled to the brim with wine, and used for toasting. Why it is called a bumper is a bit uncertain, but could be from the idea of knocking such glasses together during a toast. From Thomas D’Urfey’s Madam Fickle of 1676:

Full Bumpers crown our Blisses.

Bumper eventually came to refer to anything large or abundant. From Gentleman’s Magazine of 1759:

In some of the midland counties, anything large is called a bumper, as a large apple or pear.

By 1885 it was associated with crop, from the Times of London of 2 October:

The floods will have the effect of giving a “bumper” rubbee crop.

(Source: Oxford English Dictionary, 2nd Edition)