A punt is a flatbottomed boat with square ends. They were used in shallow waters. Whenever they came across a crocodile they would use the boat to gently hit the crocodile to convince him to move along, so their animals could cross the river safely.
In england when you argue or start a fight on some1, it's calling "having a go". Kindof like what the people did to the crocodiles in their boats, so people started calling it having a punt, and now it's changed slightly to mean having a bet something.