this is what a person does when they are in the middle of preparing a large leg of lamb for a lavish family dinner, often including in-laws. What they do is remove the bone with an inappropriately long knife for the job, creating a cavity within the meat. Once that is done, the person then pulls his trousers down and gleefully slides his privates into the cavity- thus tunnel boning the leg of lamb.
Oh man! Mark is so into tunnel boning! He does it all the time when he cooks for his in-laws!