1590s, "mute person," from dumb (adj.) + -y (3). Extended by 1845 to "figure representing a person." Used in card games (originally whist) since 1736. Meaning "dolt, blockhead" is from 1796.
In a corporation, one who stands in for a real director or who serves as a nominal director during the organization of the corporation until the stockholders can elect directors.
To keep silent; clam up: You can't dummy up on a murder case/ When questioned, they invariably dummy up (1926+)
: a dummy machine gun/ dummy windows
: The designer dummied the new book
beat one's meat