"O inglorious league!
Shall we, upon the footing of our land,
Send fair-play orders, and make compromise,
Insinuation, parley, and base truce,
To arms invasive?..."
See in text (Act V - Act V, Scene 1)
The bastard, Philip Faulconbridge, is a fierce defender of English sovereignty, but he is not consistent in his use of the term "fair play" here and elsewhere in the play. In this instance, he uses it sarcastically, indicating that the peace deal with the Vatican and the French that King John has made is cowardly. Philip equates "fair play" with capitulation, or giving up.
Wesley, Owl Eyes Editor