Green’s Dictionary of Slang

(UK juv.) a cry used to call a truce in a (fighting) game; thus pax, a friend; good pax, good friends.

1841		W.J. Neale Paul Periwinkle 465: Come, Mr. Doubtful, you’ve hit us hard there, as children say, pax!
1894		G. du Maurier Trilby 80: He [...] made such a terrific rumpus, that the whole studio had to cry for ‘pax!’.
1908		Magnet 13 June 2: Now, then, are you going to make pax?
1911		Gem 4 Nov. 20: ‘Pax!’ he exclaimed.
1945		(con. 1937) R. Westerby Mad in Pursuit 189: ‘Pax!’ they said, and it was pax indeed.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 172: Thus a 14-year-old prep. school girl in 1954: ‘When life becomes too bad you shriek “Pax”. Once you have uttered this magic word you are safe.’.
1960		(con. c.1928) D. Holman-Hunt My Grandmothers and I (1987) 165: Oh Pax! You’ll make her giddy!
1982		O. Marshall ‘The Master of Big Jingles’ in Ace of Diamonds Gang (1993) 19: ‘Pax!’ said Rainbow bitterly.
1995		A. Higgins Donkey’s Years 30: Pax!
2001		OnLine Dict. of Playground Sl. 🌐 pax, paxies n. Used in playground games. Players cross fingers and yell pax to make them safe from being tagged.