sardar jee was having arvi/arbi ka shorba with chapati
See a picture here. http://www.razatraders.com/images/arvi.jpg
If you have ever seen arvi and its salan, you will know that this is very gooey , slick kind of a vegitable and the shorba is prepared just by peeling it and cooking it whole with lots of gravey and meat.
So back to the joke , sardar jee was having trouble catching arvi with chapati morsel. It keept on sliding out , some how he caught it in his mrosel/newala and put it in his mouth and put the other hand on his behind immediately.
Now do not blame me if you do not get the joke. You will not get the joke if you never had arvi ka shorba with chapati.