A flake is someone who generally makes plans with you, promises to do things with or for you but can never seem to follow through. When confronted with their behavior they usually get defensive and run away.
Jane doe promised to go out with me. She never showed up. We made plans saturday night and she came up with an excuse why she couldn't show up. Jane is a flake.
They suddenly flake off into the crowd
John just puts a good scare in their and before someone of the bystanders can even have the time to open the mouth the poor fellas flake off in a shot