late 15c., "formal document authenticated by an affixed seal" (a sense now obsolete), from Old French placquard "official document with a large, flat seal" (14c.), also "plate of armor;" ultimately from Middle Dutch, either from Middle Dutch plackaerd or via the French verb plaquier "to lay on, cover up, plaster over," from Middle Dutch placken "to patch (a garment), to plaster," related to Middle High German placke "patch, stain," German Placken "spot, patch."
The meaning "written or printed paper displaying some proclamation or announcement, intended to be posted in a public place to attract attention" is attested in English by 1550s; this sense is in French from 15c. As a verb, "to put placards upon," by 1813.
Compare plack, a low-value Scottish coin of 15c.-16c., from Old French plaque, name of a coin, literally "slab, plate, patch, veneer, etc.," from Middle Dutch placke, name of a coin, also "a thin slice."