From Longman Dictionary of Contemporary Englishdummydum‧my1 /ˈdʌmi/ noun (plural dummies) [countable] 1 for clothesDCC a model that is the shape and size of a person, especially used in order to show clothes in a shop or when you are making clothes a shop-window dummy a tailor’s dummy2 copyPRETEND an object that is made to look like a tool, weapon, vehicle etc but which you cannot use During practice runs, the warheads in the missiles will be dummies.3 doll a small model of a person, with a mouth that can be moved so that it looks as though it is talking, used for entertainment a ventriloquist’s dummy4 sports British English a move in a sport such as football in which a player pretends to pass the ball but does not, in order to deceive the other team’s players5 
for babiesDHB British English a specially shaped rubber object that you put in a baby’s mouth for it to suck SYN pacifier American English 6 stupid personSTUPID/NOT INTELLIGENT especially American English informal someone who is stupid No, you dummy. The other hand.7 card gameDGC cards that are placed on the table by one player for all the other players to see in a game of bridge → spit the dummy at spit1
for babiesDHB British English a specially shaped rubber object that you put in a baby’s mouth for it to suck SYN pacifier American English 6 stupid personSTUPID/NOT INTELLIGENT especially American English informal someone who is stupid No, you dummy. The other hand.7 card gameDGC cards that are placed on the table by one player for all the other players to see in a game of bridge → spit the dummy at spit1Examples from the Corpus
dummy• He just walked around like a dummy with the rest of us with an awkward scope on his rifle.• A dummy has all the restrictive properties of a real job but no time content.• I bundle the blind dummy out.• Babies she had seen before had fat fleshy faces that spread from a central dummy.• She's no dummy.• a dressmaker's dummy• That's what I just said, dummy.• It was a perfectly ordinary leg from a shop dummy.• The dummy took three weeks' work by a team of volunteers, mostly part-time and helping when they could.• Briers threw two dummies and sprinted outside Henry Paul for his second try.dummydummy2 adjective [only before noun] PRETENDa dummy tool, weapon, vehicle etc is made to look like a real one but you cannot use it SYN replica a dummy rifleExamples from the Corpus
dummy• Semionov threatened the pilot with a dummy hand grenade and forced him to land the plane.• By then Ward was coming down on to the track, his false arm and dummy hand hanging limp at his side.• He claims that his dummy head does likewise.• It is a particular example of a no-operation or dummy instruction.• The candidates had been asked to supply a dummy paper with a lead story and some idea of content.• dummy rifles• Grayson dropped a ball, Kevin Maggs ran on a dummy run from the scrum and Catt was through for the try.From Longman Business Dictionarydummydum‧my1 /ˈdʌmi/ noun (plural dummies) [countable]1MARKETINGa product that is made to look like a real product and is used for testing, obtaining people’s opinions etc2MARKETINGa test advertisement or design3a model of a human used for showing clothes in a shop window or for testing car safetydummydummy2 adjective [only before a noun]1MARKETINGa dummy product is made to look like a real one and is used for tests, getting people’s opinions etc72% of the men in the group taking a dummy pill continued to lose hair.2ORGANIZATIONSa dummy organization is used to hide the real owner of assets, or to hide criminal activitiesThey had set up a series of dummy corporations to buy and sell 8,000 acres of useless New Mexico desert.3a dummy activity is used to make people think that something is happening when it is notThey used dummy trades to create a false impression of a more active market than really existed.Origin dummy1 (1500-1600) → DUMB1 