Just had my first app idea request from a friend. How do I tell them I'm not interested while being sincere?

“Thanks for thinking about me, but I'm not really interested.”

Does it already exists and you just want your name on it?

Yes?

No!

"I'm sorry, I really don't have time, your idea sounds great, maybe you should ask X" (blame shifting in another friend you don't get allong with that well)

It takes a lot of time to develop an app, it doesn't magically appear out of nothing. I'm not interested enough to commit to that

Tell them yeah, you should learn "this, this and this" technology and I might help you make it.

That's what I usually do, success rate 100%

I usually dodge these with "You know I have a fulltime job right. Besides that even if we have the application it wont sell itself and it will take a lot more to grow a product." .

Then I proceed explaining how the company I work at had a great idea, built the application but failed to market and sell. In the meanwhile competition gained the advantage on us forcing us to quit.

Go into great detail of the technical process and halfway through mention cost. Works for me, I never get asked again (unless they can pay for what they need)