What's this about?

From Englishman John Brophy's semi-autobiographical novel of WW1, "The Bitter End" (1928), referring to 1917-18:

"There was current at the time a slang phrase used to express intense pleasure and triumph and accompanied by a gesture of the fingers. Everyone not too genteel used it, in this sense, but among coarser and more knowing people it passed as a euphemism for the sexual act performed illicitly....The soldiers were of course aware of the double meaning."

Q: What slang phrase? What gesture? (The OK sign? Beats me!)

Anybody know?