Green’s Dictionary of Slang

the penis.

1842		Flash (NY) 31 July n.p.: Norwich, Conn. Wants to Know [...] What Jack H was talking about with that black woman [...] was it concerning that locomotive with the piston which works through the boiler.
c.1886		Tilly Touchitt 42: Passing up the slippery sheath thus newly opened [...] piaston and socket work with a vengeance.
c.1900		‘Ramrod’ Nocturnal Meeting 40: Nothing but a smooth elipse of flesh could be seen with the great piston boring into the centre.
1966		Trimble 5000 Adult Sex Words and Phrases.
1978		L. Kramer Faggots 321: Peter, piccolo, piston.
1983		R. Price Breaks 267: Kiss my piston.
2002		Bill Healey ‘U4ME’ [poem] on Originality 🌐 Evicting testicular squatters, giving the piston a lube / Squeezing the toothpaste in the middle of the tube.