Something you call a person when they're acting very immature or childish.
"egg water" refers to the liquid in an egg (the yolk or the white), calling a person "egg water" implies that they're acting like as if they just recently hatched from an egg (a.k.a they're acting like a baby or immature kid).
The word is also a twist on the word "dog water", replacing "dog" with "egg" to give it a slightly different meaning.
Example 1
Friend 1: "Waaaahh! This is NOT FAIR! This game sucks! Stop being better than me!"
Friend 2: "Man, stop being egg water, it's just a game."
Example 2
Friend 1: "Dude, I've got the most genius idea." Proceeds to draw a giant penis on a wall with graffiti.
Friend 2: "Dude, this is so egg water, couldn't you think of something like a joke with a punchline?"
The water that hardens the egg softens the potato is a Russian saying that means everyone reacts to stressful circumstances differently. It is often used in the context of trauma.
"Rachel's mom just died and a week later she's back to school. Meanwhile, I'm strugling to get out of bed, just because my hamster died. I feel so shitty."
"Dude, don't beat yourself up about it. The water that hardens the egg softens the potato. "