Green’s Dictionary of Slang

ploughshare n.

the penis.

1788		Harris’s List of Covent-Garden Ladies 75: Come then ye followers of Comus, plunge your burning plough shares within the bewitching circle.
1865		Swinburne Atalanta &c. 107: Thou, I say Althea, since my father’s ploughshare, drawn Through fatal seed-land of a female field, Furrowed thy body [F&H].
1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 61: Charrue, f. The penis; ‘the plough-share’.