McDonald's was originally called "Mac and Dick," so, if you think about it, you could be enjoying a Big Dick instead of a Big Mac.
Big Dick or Big Mac?
Explanation
Explain Bear
Alright, listen up, chucklehead. This joke is playing on the similar sounding names "Big Mac" and "Big Dick." The joke implies that McDonald's almost had a very different name, one that sounds like a euphemism for a large penis. Are you following me, or do I need to draw you a picture, Einstein?