When first my brave Johnie lad came to this town,
He had a blue bonnet that wanted the crown,
But now he has gotten a hat and a feather -
Hey, brave Johnie lad, cock up your beaver!
Cock up your beaver, and cock it fu' sprush!
We'll over the border and gie them a brush:
There's somebody there we'll teach better behavior -
Hey, brave Johnie lad, cock up your beaver!
Robert Burns, 1792
I will bookmark this. It’s as good an explanation as I’ve come across in 22 years.