Stale Piss in a Bottle

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In my dreams people piss on me all the time, but it never happens in real life.

One of my friends from primary school, his father used to tell him he’s as “useful as stale piss in a bottle”.

I’ve never really understood this saying. Does it mean that fresh piss in a bottle is useful? And if the piss were contained in something other than a bottle, might it be useful then?

The main point is that his dad thought he was useless, and whether you reckoned him fresh or stale, bottled or unbottled, useless is useless.