'Whistle for it' is an idiom. If someone says that you can 'whistle for something' they are determined to ensure that you don't get it. Example:If they want another salary increase, they can whistle for it. They're not getting another cent out of me!

'Whistle' is the musical sound you make when round your lips and blow. You can train your dog to come to you when you whistle. The phrase 'whistle for something' is mainly used in the rather old-fashioned idiom 'He can whistle for it', which means that you have no intention of giving someone what he has asked for. For example. 'John expects me to give him money for that stupid thing he's bought. Well, I'm not going to. He can whistle for it!'.