Dummy has several meanings:

1. A model or figure made to look like a person, used for a specific purpose. Store windows display clothes on dummies (also called mannequins) so shoppers can see how outfits look when worn. Crash test dummies help engineers test car safety without risking human lives. Ventriloquists perform with wooden dummies that appear to talk and tell jokes.
2. In card games like bridge, the dummy is a hand of cards laid face-up on the table for everyone to see. One player controls both their own cards and the dummy's cards, making strategic decisions for both hands.
3. A practice version of something real. Football players tackle padded dummies during drills. Publishers create dummy books (blank pages in a cover) to test how a book will look and feel before printing the real version.
4. An insult suggesting someone is unintelligent. This usage is unkind, so it's better to avoid it.