It seems to have come down from C16th Scotland, where pow/poll meant head, and is mentioned in this song. There's quite a few other people providing evidence that pow is Scottish for head, on the linked site.
Dainty Davie
as recorded by
Finbar and Eddie Furey
In yonder town there lived a maid
Ah, she was the keeper of her trade
She fell in love with a light brigade
And his name was Dainty Davie
Chorus [after each verse]:
So, leeze me on your curly pow
Dainty Davie, Dainty Davie,
Oh, leeze me on your curly pow,
You are my Dainty Davie.
My love, my love, my love,
brine he
I have the longing for the sea
I'll go defend my country
Say farewell to Dainty Davie
Three days and seven years were gone
When she saw this figure came striding on
Oh she knew it was her own true one
Saying, come kiss me, Dainty Davie
http://www.mudcat.org/thread.cfm?threadid=9055