Other forms: bumpers
A bumper is the bar at each end of a car that takes a small knock. Designed to absorb the shock so the rest survives.
Modern ones are moulded plastic over a hidden crash structure and do far more work than they appear to. Bumper-to-bumper traffic is nose to tail, and a bumper sticker is a message stuck on the back. There is a completely separate adjective meaning unusually large, so a bumper crop and a bumper issue are both good news, from an old word for a brimming glass.
From bump + -er.
Etymology from Wiktionary, CC BY-SA