Hudibras is an English mock heroic narrative poem written by Samuel Butler in the 17th century. The quote refers to the following verse:
Which on his rider he would flirt
Still as his tender side he pricked
With armed heel, or with unarmed, kicked:
For Hudibras wore but one spur,
As wisely knowing could he stir
To active trot one side of's horse,
The other would not hang an arse.