Codeword for marijuana. It is generally used with the article attached.
Stoner 1: Do you want to have the fun?
Stoner 2: Totally!
- A short while later -
Stoner 2: Man, this fun really fucked me up.
Stoner 1: When you have the fun with me, you know it's gonna be the good shit.
Here is fun in yer eye!
Grab the fun.