Yah Boo Sucks is a not very polite expression, meaning something like "put that in yer pipe and smoke it" or "I am right and you are wrong. So there!"

You had said Praise's new name was unanagrammable and Steve gave you an anagram of Praise.

But Steve didn't know you meant "WYSIWYG" not "Praise". So you were right and can tell Steve "Yah boo sucks to you, too!"