Based on the legal code, around here it's slang for "crazy".

Poster: "I was ripping my dirt bike out in the snow and got pulled over!"

Commenter 1: "What for?"

Commenter 2: "5150."

Barcroft Station in California (elevation 3800 m) has house number 5150 over the door. You'd have to be crazy to want to work there.

In my GenX brain, a Van Halen album* comes to mind.

* https://www.youtube.com/watch?v=OFxg1nB9yQ8

I had the same question. It is the title of a Van Halen record album, also a section of the California legal code related to mental health, according to a simple search.

As a Van Halen fan, I think of it because of the album titled "5150". But it has other well known uses as well.

It's apparently the California law section number for restraining a mentally unwell person or something, so has law enforcement and slang usage, and there's a 1986 chart-topping song named after it. (I'd never heard of it either, but I'm not Californian.)

This being HN, my mind went immediately to the IBM 5150...

https://en.wikipedia.org/wiki/IBM_5150

This being HN, there's only a few of us left.

One of the best Van Halen albums?

And Eddie’s amp model, the Peavey 5150. It’s become the de-facto standard for the more extreme metal bands.

When Eddie took the rights with him to Fender and they made the EVH 5150 (another fantastic amp), Peavey renamed this line to the 6505 series, so there you have another four-digit code to use.

Well the 5150 was Eddie’s Studio Postal Code, but if we are going down this road :-) I would like to point out at some point in time they run out of Sylvania 6L6 Power Tubes... then Peavey started with Chinese Ruby 6L6 Power Tubes for the EVH5150 and they dont sound the same...

This is one of the first results a search provided. I skipped Wikipedia which was first but it was actually informative as well.

https://www.dictionary.com/e/slang/5150/

Color me a bit sad to not see 2112 make the list.

Its the famous law for involuntary mental lockup in California, then referenced a lot in pop culture, probably most notably with a Van Halen album named after it. Its used in a lot of jokes, but also oppressively. I think we've seen some divorce court releases and such on how to "5150 my wife," how cops abuse it, etc.

Around these parts, we call it an "M1 hold"

M1 is - I think - the form they have to fill out to put you on a temporary hold.