You can find all sorts of weird things on the web ... including Answers.com's made-up version of where the Dumb Polak stereotype comes from.
Where do people come up with this stuff? And why do they pretend that they know something when they don't?
Luckily many people are aware enough not to believe everything they read on the web.
You can read the full Answers.com invented history of Polak jokes here.
Here is an excerpt:
"The phrase 'dumb Polack' generated after WWII. When Hitler's army invaded Poland, the Nazi soldiers were ordered to hunt down and kill any person of intelligence in the country. This included: authors, painters, teachers, and any other person with a high level of education. With all the intelligent people of Poland murdered, the phrase came to be because the "dumb" citizens were left to pass on their genes to future generations. Sad but true."