"Red-haired of both Sexes are more libidinous and mischievous than the rest, whom yet they much exceed in Strength and Activity"

 

In the UK red hair is generally associated with people of Celtic descent, i.e Scotland and Ireland. It is believed the people of Scotland came from 5 different ethnic groups who occupied or invaded northern Britain in the dark ages. In all of recorded history, red-haired people have never been mentioned as a group except by the Romans.

The 'Picts' where foes who the Romans fought and were described as having red-hair and 'large limbs' by Roman historian Tacitus. Modern historians with the help of anthropologists have placed red-hair as a unique characteristic belonging to the Picts, who were characteristic to what is now regarded as Scottish.

Either Swift is denigrating the Scottish since they have had, historically, red hair. Or he is bragging because he is Irish and they are included in the "red-haired" classification.