The only reference I can find on the net to the origins is this In Britain,
for instance, you may be confronted by people who pinch you, then punch you
on the arm, and say "Pinch, Punch, First of the Month." The origins of this
rather unpleasant custom are lost in time, however I can offer you some
useful advice. The only defence against this custom is to pinch and punch
first because tradition demands that the pinchee is not allowed to
retaliate.