clicker (n.)

"one who or that which 'clicks,' in any sense," agent noun from click (v.). Earliest attested sense is slang, "person employed by a shopkeeper to stand at the door and solicit customers" (1680s). It is still thus in Johnson (1755): "clicker. A low word for the servant of a salesman, who stands at the door to invite customers." By 1996 as "a remote control device to operate something (generally a television set) by the 'click' of a button."