What does Dean mean in S2 E6...

cleaning the pipes = jerking off
she squeezed by him, tight space, do the math.

A guy can get an erection easily and in the tight space they were in, with Jo rubbing against him, he obviously felt horny and needed to masturbate, but he says "Should've cleaned the pipes." As in I should have masturbated or had sex before now.

he means the PIPES, duuh! no, srsly. I dunno...

he means he should have gone to the loo so he wouldn't be as bloated and he might have been able to fit through the gap :P

A lot of this show has double en tundras. This is one of them. It could mean either going to the bathroom or having sex. Just one of the great things about his show is the script

It's a slang term for masturbation. At the point Dean utters this, Jo is squeezing past him, brushing very close against him, to get by. Since she is a gorgeous young lady, and he's just a man, it's uncomfortable and he may be embarrassed by having an erection. When caught saying this, he claims it's about the real pipes in the wall, but she's not that stupid. It was basically a vulgar joke that could pass on TV.