"a laughable idea" is correct and usable in written English.
You can use this phrase to describe an idea that is ridiculous or absurd. For example, "My brother's suggestion that I should train a monkey to mow the lawn was a laughable idea."
It makes things feel adversarial, as if a family's agreeing on what to hang in the dining room is a laughable idea.
At the same time, I think that the kinds of people who were uncomfortable with the Affordable Care Act, who were uncomfortable with some of these more universal health care systems in the past think it's sort of a laughable idea.
"I think delivery drones is a laughable idea," adds Caccia.
The Sepracor bonds can be converted into 15 shares of stock, a laughable idea, given that this many shares are currently worth $195.
Why would they risk their personal brands to appear in something promoting such a laughable idea?
Considered in the abstract, the idea of poisoning our food and the planet that sustains us would be a silly and laughable idea, but it's not abstract, it's real, it's happening now and it's deadly!
We should rise above her laughable idea that freelancers are "born with a special kind of immunity to sickness".
Let's leave aside the laughable idea that these individuals will operate independently in a non-partisan manner, for the good or our beloved country.
Just when you thought he couldn't get any more imaginative, there's another laughable idea here.
The laughable idea of the Tom Skerritt "High Noon" merely hints at the many problems with this movie, which doesn't begin to do what good remakes can: reinvent a film for a new generation.
