How to open jammed and rusty nut from the water stop cock?

[xjadeaurealis]

I've tried WD40 and waited for about an hour before trying again to loosen it but it's stI'll does not move. I adjusted this stop cock to do a pipe adjustment but after touching it, it can no longer turn on. So right now all outlets connecting to this stop cock cannot use.

 

[Wayners]

Need to get something with a tight grasp on nut and smack it. The shock of hitting will crack seal/rust.
Get a spanner that's nice and tight or a quality adjustable and give a good smack with hammer.

Could you cut pipe and get a socket on nut?

6 point socket not those 12 point star sockets that round off corners. Want a full 6 side contact

 

[JohnD]

What's that is projecting through? Is it the back of a kitchen cabinet? If a tile, you may have to sacrifice it.

You could cut a larger opening to give you more working room, and fix the panel back afterwards.

A spanner that is a perfect fit is better than a sloppy one, but you need to brace the pipe. If it is copper it may twist and fold.

 

[flameport]

Could you cut pipe

It's not a pipe - it's the spindle and packing nut of a stopcock that someone has buried behind tiles.

I've tried WD40 and waited for about an hour before trying again to loosen it

If the handle doesn't turn, then it's jammed internally and will require dismantling. Possibly replacing entirely.
Spraying substances at it will not help.

Whatever that tile or panel is that it's projecting through will have to be removed.
You will also need to turn the water off somewhere else.