What expression do you think should replace this outdated one?

Ass backwards

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Buying a car before you have a license.

“Buying the app before the device” could work as a thoroughly modern replacement, though it will probably become outdated someday as well. A more generic phrase would be “getting ahead of yourself.”

Hate the president before the inauguration.

Looks like we need a new term for “beating a dead horse,” too.

Hanging up the phone.

Here’s one that will be contemporary for about another 5 billion years:

You can’t celebrate Sunday before our star is born.

Who said it was out dated??
Go off half cocked.

Putting the chauffeur in the tractor

Trump is evil, @Patty_Melt. I know you’re not stupid, which is why it’s so strange to me that you don’t understand that.

@cinnamonk Moreover we knew it before he was inaugurated.

Put the trailer in front of the truck.

Having the divorce before the marriage.

I got the impression he was a jerk sometime in the 80’s. It was solidified in 2004 when he tried to trademark the phrase, You’re fired!