"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.
"According to the fair play of the world,
Let me have audience..."See in text(Act V - Act V, Scene 2)
In this second instance of Philip's use of the term "fair play," he uses the term to seek an audience with the Pope's legate as courtesy and chivalry demand. However, he still expresses some sarcasm, because the point of seeking this audience is to reject the peace deal with the Pope and the surrender to France. He uses "fair play" as a customary courtesy, a show of civility, to indicate that he desires a peaceful audience—even with those he may hate enough to harm. This expression represents a mark of civility for us (like playing by the rules of the game), but Philip considers it merely an ambivalent quality, a not always necessary evil.