A above-avrage to huge penis. Usually a second or even third word is added. The combination of the different words increase the symbolism of the expression.
Often used combinations:
"steel pipe", "hard pipe", "cum pipe", "fuck pipe", "hard steel pipe",...
I'll fuck you with my hard pipe.
As I watched her finger her wet pussy my dick grew to a hard steel pipe.
That slut deepthroated my cum pipe the other day.
She sucked all the cum out of my pipe.