Bumper
early 17th century (in the sense ‘person or thing that bumps something’): from bump + -er1. bumper (sense 4 of the noun) derives from the earlier form bumping, meaning ‘very large, great’, and is the source of the adjective meaning ‘exceptionally large, fine, or successful’, as in a bumper year . bumper (sense 3 of the noun) is said to be from an earlier racing term meaning ‘amateur rider’.
wiktionary
From bump + -er.
etymonline
bumper (n.)
1670s, "glass filled to the brim;" perhaps from notion of bumping as "large," or from a related sense of "booming" (see bump (v.)). Meaning "anything unusually large" (as in bumper crop) is from 1759, originally slang. Agent-noun meaning "buffer of a car" is from 1839, American English, originally in reference to railway cars; 1901 of automobiles (in phrase bumper-to-bumper, in reference to a hypothetical situation; of actual traffic jams by 1908).
