Toon, toonh, or Tooh as it's spelled by the makers of Gori Tere Pyaar mein, is a Punjabi word, more of an unofficial kind, which is used for bum, ass, backside, behind, whatever you call it. In fact the usage of the word here is cute kind, like sometimes tush is used.
Now if you would want to know what's an 'unofficial' kind of a word, let me tell you that the word is not easy to find in a dictionary, and I couldn't find it online or offline in a dictionary. However, the lyrics very clearly tell in the first line itself that the word is used for 'booty'. So looks like it's one of those words that pass on through generations unofficially, without being formalized. Also, words like 'toonh' would not have too high a possibility of being formalized I guess, so unless it gets more official, let's shake our toonhs unofficially. :D
For the meaning of entire Tooh song, check THIS POST.