It's not much of a leap. "Stupid" is a judgement of intelligence. Ideas don't have intelligence, people do. To say something is "stupid" is to say that's it's

It's not much of a leap. "Stupid" is a judgement of intelligence. Ideas don't have intelligence, people do. To say something is "stupid" is to say that's it's the product of unintelligent thinking. Somebody calling my idea "stupid" may not be saying that I'm universally stupid, but they're at least saying that I was momentarily stupid to suggest that idea. Same thing if you call my idea "retarded" or "idiotic", or conversely, "smart" or "clever". The argument could be made that we should all be rational beings of pure logic, incapable of taking personal offense or being emotionally slighted or discouraged by such judgements. But we're not. And if we were, we would also be capable of limiting our output to objective conclusions such as "that won't work" rather than subjective judgements such as "it was foolish to suggest that it could".