The verse you are asking about goes like this:
You might go to sleep without punching the bag.
You might raise a chicken without laying an egg
But you can't raise a cow from the calf of your leg
Don't try it for it can't be done.

Turns out that folks used to use the following expressions for going to sleep:
"I am going to punch the ol' bag."
"I am hitting the bag"
"I think I will punch the sack."
"I am going to punch the sack."

Also the very first verse begins:
"Folks, never say - I am the best"