Green’s Dictionary of Slang

jug-a-jug-jug v.

[north/midl. UK dial. jug, of partridges, to roost close together]

to copulate.

[UK]J. Meades Empty Wigs (t/s) 285: he used to jug-a-jug-jug with so many different creatures... [...] oh he was such an irrepressible animal lover..