When someone passed around an Elvish joke on Tumblr, tseecka at My Own Little World decided to set the record straight. She goes on to explain exactly how one really would say "Can I touch your butt?" in Elvish, and why.
That's one thing I love about the internet. No matter how obscure the subject, there is always someone with extended knowledge who is willing to share it with the rest of us. Buzzfeed has a roundup of instances where commenters added useful, mind-blowing, and possibly even factual explanations for viral images in 17 Times Tumblr Explained A Thing Better Than School.
Assuming the context, it should be "May I touch your butt?"
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