sandwich board

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A pair of large placards, hinged at the top by straps for hanging over the shoulders with one board in front and the other behind, used for picketing or advertising.
• noun A pair of signboards fastened at their tops and separated at their bases to form a freestanding structure, as for sidewalk or roadside advertising.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person-sized, wearable billboard or advertisement consisting of two printed boards strung together so that they hang over the shoulders, thereby sandwiching the wearer.
• verb to advertise or campaign using a sandwich board; to wear a sandwich board.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun signboard consisting of two hinged boards that hang front and back from the shoulders of a walker and are used to display advertisements