I’m sure it started when a Pole named Nicolaus Copernicus published his hairbrained theories about the earth orbiting the sun.
Anyone with half a brain knew the earth was fixed into place - and the sun spun around in the sky over us.
Such a ridiculous idea gave the entire Polish nation a bad name…and that ham pianist Chopin, or that Curie woman added to the stereotype of the dumb Pole. I mean radium…c’mom…a trained seal could have discovered that!
The ironic thing is that, even though Bletchley Park and Alan Turing get most or all of the credit, the first (and you might say most important of all) major breakthrough in cracking the Enigma cipher was made by the Polish.
One of the most important military ANTI-fuckups ever.
http://www.pbs.org/wgbh/nova/transcripts/2615decoding.html
Forgot the link. 
A previous thread on Polish jokes: http://boards.straightdope.com/sdmb/showthread.php?threadid=36844
A couple of pages that examine the story of Polish cavalry allegedly charging tanks:
While Americans tell Polack jokes and Canadians tell Newfie jokes, in India they tell Sikh jokes.
A guy is walking in the country and sees a Sikh lying on the ground next to a cow. He says, “Can you tell me what time it is?” The Sikh reaches out and pushes up the cow’s udder and says, “It’s five minutes till noon.” Impressed, the guy says thanks and goes on his way. Later he’s going back that way and again asks the time. The Sikh again lifts the cow’s udder and says “It’s half past noon.” Amazed, the guy asks, “How can you tell the time by doing that?” The Sikh answers, “When I move the udder out of the way, I can see the clock tower over there!”
Oh, but they are. That is not to say that every generation in the US is growing dumber - most are barely able to write or speak - but there’s a special sort of “stupid” Polish people exhibit! You’re not Polish are you? The correct spelling of the third-grade vocabulary word that you used, is “they’re”. Not Polish? By chance, were you born after the 1970’s?! 
Some might say that posting in a thread 21 years after it was started, correcting the grammar of a poster who hasn’t been here for 21 years, is not enormously clever.
Moderator Note
Dredging up a 20+ year old thread just to insult people based on their ethnicity is not a good way to introduce yourself to our message board. Attacking people based on their ethnicity is not permitted here. Insulting other users is not permitted here (except in the BBQ Pit forum). Do not post anything along these lines again or your posting privileges here will be revoked.