n. a car or other vehicle following too closely on one's bumper. (A reapplication of the term for a kind of adhesive sign stuck on a car bumber.) : I can't talk now, I've got a bumper sticker that's taking all my attention.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw-Hill Education.
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It's not the kind of thing you can put on a bumper sticker, either.
It acted as a hugely successful bumper sticker, long before there were bumpers.
All orders that have been filled for hats, shirts and parks gift certificates also have received a free bumper sticker.
Outreach and education is more than slapping a slogan on a bumper sticker.