Green’s Dictionary of Slang

to have sexual intercourse; thus furgler/fergler n.

1923		in E. Cray Erotic Muse (1992) 230: They fergie [sic] a bull and fill him full / those hardy sons of bitches.
1947		C. Willingham End as a Man (1952) 83: A natural-born fergler.
1966		(con. 1948–52) L. Thomas Virgin Soldiers 21: A harmless mechanic [...] quietly furgling another WRAC in the long elephant grass at the side of the garrison road.
1970		Current Sl. V:3 6: Flurgle, v. To have sexual intercourse.