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" is from 1759, 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).