Wank in the Bush
When you have an awkward boner and cannot keep it in your pants any longer and there isn't a bathroom near by, so you whip it out and pretend you're taking a piss but secretly wank in the bush.
Dan: "Liam mate are you having a wank in the bush?"
Liam: "Na mate just taking a piss, being holding it in all day!"
Liam: "Na mate just taking a piss, being holding it in all day!"