The Joke:
What do you call a person with no body and no nose?
Nobody knows.
The joke asks what you call a person with:
No body: Literally, they lack a physical torso and limbs.
No nose: They lack the organ used for smelling.
The Wordplay:
When you say the punchline "Nobody knows" out loud, it functions as a double entendre:
If you have no body and no nose, you are—quite literally—"no body nose."
"Nobody knows": This is a common English idiom meaning "no one has the answer" or "it is a mystery."
The humor comes from the fact that the answer sounds like a standard conversational phrase, but it is actually a literal description of the ridiculous person described in the setup.