Longman Dictionary of Contemporary English
Wiktionary
n. 1 An adhesive label or decal, usually applied to the bumper or back end of an automobile, and displaying a short political, promotional, philosophical, or humorous message. 2 Metaphorically, said of canned insight; a slogan or saying that is trite or lacks depth.
Wikipedia
A bumper sticker is an adhesive label or sticker with a message, intended to be attached to the bumper of an automobile and to be read by the occupants of other vehicles—although they are often stuck onto other objects. Most bumper stickers are about 30 cm by 8 cm (12 in by 3 in) and are often made of PVC.
Usage examples of "bumper sticker".
Regina threatened as she noticed a white minivan with a rainbow bumper sticker parked in front of a small brick building that said BAPTIST CAMPUS MINISTRY out front.
We raised our hands as they drove past in their cars - Huddie's big old New Yorker, Shirley in her little Subaru with the bumper sticker reading MY KARMA RAN OVER MY DOGMA.
The retards get out at the same time as the kids at Derry junior High, but most of them go home with their mothers on the special retard bus, which is blue instead of yellow and is reputed to have a bumper sticker on it that says SUPPORT MENTAL HEALTH OR I'LL KILL YOU.
A bumper sticker on the back deck of the Ford's trunk read: GET YO ASS UP THE PASS--COLD CREEK CANYON.
In fact, she was ambivalent to a degree that made her wonder how her bumper sticker would have read had one been affixed to her buttocks on that nippy January afternoon.