Learner's definition of UPSIDE DOWN
: with the top at the bottom and the bottom at the top : placed so that the end that should be at the top is at the bottom
You hung the picture upside down!
To remove the plant, turn the pot upside down and tap gently on the bottom to loosen it.
The baby was holding the book upside down.
turn (something) upside downinformal
: to make (something) very untidy — often used figuratively