Adverb

1. Inverted, so that the top is now at the bottom

   The Union flag was flying upside down, a sign of danger.

2. In great disorder

   The thief had turned the room upside down.

Adjective

1. Inverted; the wrong way up.

   Would you like some of my upside down cake?

2. Owing more money for something than it is worth; having negative equity.

   He's upside down on his mortgage.