Well, a good definition for the term would be an idea that would prevent anyone holding it from functioning in society. And, in this, I am not talking about ideas so shunned by society that one must feign disbelief in order to function. Perhaps a better definition would be an idea that one would confirm to be false in one's day-to-day operations. In this definition, the notion that mice eat cats would be ridiculous (for anyone with experience with the two animals.)
In my observations, when people call an idea ridiculous, they normally do so unfairly. It is generally an idea that they wish to dismiss without thinking about it. Simply put, people don't like to consider worldviews other than their own.