English to Turkish Art/Literary Poetry & Literature Novel

Can you explain "hard-ons" in this sentence. I tried to saya "erection" and "diffciulty" but neither seemed to fit the sentence.

I'm lying in the front seat of the Boomerang with my trousers round my knees and copping a twenty-dollar blowjob from a speed-fuelled Zulu called Agnes. I mean it's incredibly reasonable, don't you think? What a fine country. What value. With sterling in the shape it's in, that's barely nine quid! But Agnes and I have a problem. 'This is why they're called hard-ons,' I remember explaining to her. 'They're not at all easy. They're very difficult.'