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.)). The meaning "anything unusually large" (as in bumper crop) is from 1759, originally slang. The agent-noun meaning "buffer of a car" is from 1839, American English, originally in reference to railway cars; 1901 of automobiles, in the phrase bumper-to-bumper, in reference to a hypothetical situation (it was used of actual traffic jams by 1908).