Thanks for your explanations about "isn't half the fun".

I also searched the dictionary, and found the definition of "not half" :
not half! Exclam. Certainly! absolutely! definitely!

So "Stealing his lunch is not half the fun!" could also mean "Stealing his
lunch is absolutely fun!" Am I right?

The sentence was taken from Garfield comic strips here:

http://www.ucomics.com/garfield/2004/03/17/

Await your replies, thanks!