Be sure you know what something says before you have it permanently placed on your body.

Language Log finds a wildly mangled bit of "Latin" on someone's back and comes up with the chain of events that led to it.

I figured it couldn't just be Chinese characters people were misusing.

The scary thing is that there may be more people who think "Cemel Dosce" means "know thyself" than there are people who have read the Wikipedia article for the phrase, which gives the aphorism in both the original Greek and in actual Latin:

It is unlikely that it was OP himself who did this misreading. In fact, there are 3,460 results for this phrase as well as at least one other tattoo based on this misreading.

Maybe some of them will now try to edit the Wikipedia page.