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The act of pouring piss into ones anus with a funnel, after which masturbating furiously for 10 seconds (acting like the pump on a water pistol). After which the participant squeezes the piss out of their arse like a water pistol.
''Last night my friend came round to my house and we both help each other fill out water pisstols then had a water pisstol fight''

''Last night I was practising with my water pisstol, I shot three meters''
by Wateryanus September 10, 2012