Green’s Dictionary of Slang

bumper n.2

[the bumping of glasses in the toast or f. SE bumping, huge, great. Popular ety. suggests a supposed Fr. toast, au bon père, to the good father, i.e. the pope]

1. [late 17C–19C] a full glass, esp. when raised in a toast; also as v., to make a toast; thus bumpering n., drinking, bumper, to drink ( a toast).

2. attrib. use of sense 1.

3. [mid-19C+] anything unusually large or plentiful.

4. [1940s+] (W.I.) a drunkard, a habitual drinker, esp. of rum.