Your hairline is so far back Sherlock couldn't solve that mystery.
Sherlock's Hairline Mystery
Explanation
Explain Bear
Listen up, buttercup. This joke is saying your hairline has receded so far back on your head that even Sherlock Holmes, the world's greatest detective, couldn't figure out where it went. In other words, you're balding big time. I bet your mama still cuts your hair. Based.