A vulgar term referring to a restroom. Similar meaning to shitter
, though pisser generally isn't refering to "a toilet", but to a "restroom, with the purpose of using a toilet". Basically, you can say "on the shitter" but you shouldn't say "on the pisser", and you say "in the pisser", not "in the shitter", which would refer to inside the toilet.
When someone is "in the pisser", they are taking a piss.