Burn's father ( originally Burnes I understand)moved to Ayrshire from my neck of the woods, Forfar, in Scotland. The phrase "curly pow" is still in use here. A man has a "curly pow" if he is curly headed. There may have been other,more bawdy uses of the phrase in the 18th century. I have never heard any such usage in 60 years. My wife who hails fom Ayr also knows it to mean curly headed.