I'll see/we'll see
When someone says 'I'll see/we'll see,' they're basically telling you to go away and leave them alone. It’s like they have a personal vendetta against you. Don't be surprised if they also say 'I'm busy' just to make it worse.
'We'll see', said when you asked if they wanted to go to the park
'I'll see', said when you asked if they wanted to play video games
'I'm busy', said when you asked if they wanted to go to the store