Co znaczy Szalejesz ?

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It may also mean something like partying, depends on the context.
"Dziewczyny szaleją na parkiecie." - It would mean that they are at the dance floor and they're having great party time.
Or like a parent can say about their kinds having a sleepover: "Dzieciaki szalały całą noc." - "The kids were having fun the whole night like crazy"

Other than that also, szaleć can also mean getting out of control in some way. You're going shopping and you're planning to spend a lot of money, and your friend can say, "Nie szalej tak. Nie stać nas." - Don't get out of control, we can't really afford it.
You buy a brand new car and your friend goes, "Ale szalejesz!" it's like you're going reckless, here: with money.

When you make it into szaleć za then it means being crazy about someone.
"On naprawdę za nią szaleje. Zakochany jak wariat." - He's really crazy about her, crazy in love."

It's different than oszaleć. Oszaleć means being crazy as being madly angry or in doing something dangerous.
"Oszalałeś? Chcesz nas zabić?" Have you gone mad? Do you want to get us killed? (for example when someone drives a car in a crazy manner)