这主意真是荒唐透顶。用英语怎么说？

这主意真是荒唐透顶。

It's a completely ludicrous idea.

completely

adv. 完全地，完整地，完全，彻底，全部，一干二净，很，非常，充分，十分，万分；

例句
...a second career which absorbed her more completely than her acting ever had.
比演艺生涯更让她投入的又一职业

ludicrous

adj. 可笑的，荒唐的；

例句
A ludicrous thought flashed through Harry's mind...
一个荒谬的念头突然在哈里脑中闪现。

idea

n. 主意，想法，[哲]理念，观念，[乐]乐句，模糊想法；

例句
It is often a good idea to start with smaller, easily achievable goals.
一开始把目标定得低一点、容易实现一点，总不失为一个好主意。