69 pee when you do kind of sick
You have the last two letters out of order, so there's that.
In gaming circles, "pwnd" is an intentional misspelling of "owned", as in utterly defeated. So, "pwndu" is "owned you" in gamer parlance. I have seen references to that combination of letters for many years now, so I can confirm the veracity of it.
Now, why the 69 is in front of it I can't say - I'm not convinced by the explanation given in the article about the plate owner being an astronomy buff and first moon landing being in 1969, but whatever. 1st amendment should trump anyone's feelings over any offense at the plate, whether real or feigned. Shoot, I'm betting there are state-issued plates with some variation of 69 either followed or preceded by a character combination that would be considered offensive by some.