If the idea is extremely simple and your team has talent then you could do a better job with someones idea than they could.

I think build out is perfectly valid way to explain building more functionality on a idea.

google didn't build ad functionality into from the start their search engine it was built out afterwards. I come from a family of builders so this makes sense to me.

I also don't agree with being secretive, ideas are a dime a dozen so no need not to share.