half-baked idea

1.an idea which is barely thought out.

2. an idea which sounds good, before one works out the intricacies.

3. a idea chosen for humor rather than feasibility.

Created by Bill Simmons

Jim: We should create... heroin bars!
Jack: Have you thought that through? How would it work?

Jim: I guess we would model them on hookah lounges. I don't know, it's a half-baked idea!