qoutring from chapter one:

When I first heard his name, I said, just as you are going to say,
"But I thought he was a boy?"
"So did I," said Christopher Robin.
"Then you can't call him Winnie?"
"I don't."
"But you said--"
"He's Winnie-ther-Pooh. Don't you know what 'ther' means?"
"Ah, yes, now I do," I said quickly;
and I hope you [referring now to readers of this book] do too,
because it is all the explanation you are going to get