A P. Because it's like an ARR, but it's missing a leg.
He lost it back in the 'Nom War
Bob.
They always miss the shot
Your mailbox is missing!
Pneumonoultramicroscopicsilicovolcanoconiosis
Their P is silent.
What is a pirate's favourite letter
C
the pirate replies: "Arr, it's been driving me nuts"
Arr! It's driving me nuts!