They say people are 75% water.
But I’m 75% an orphan and 25% useless.
Alright listen up, buttercup. So, the joke plays on the common factoid that humans are mostly water, right? But this chucklehead twists it by saying they're mostly an orphan and useless. It's funny because being an orphan isn't a measurable composition like water, and 'useless' is just a sad way to describe themselves. Also, you are useless.