replacement ball joint question

[VFDPrim]

replaces my ball joints this week and everything went better than I thought it would however there is one thing I'm not sure about.
the nut on the lower ball joint that's meant to hold it on is some type of lock nut and only barely gets on as when you try to tighten it the bolt of the lower ball joint just spins. it does this for air gun or hand tightening it.
the nut basically has 3 pinched points on one side of it so it won't go up the threads anymore.

why is this?

how should it be?

what should I do?

thanks for any help in advance.

 

[Dodge 1500 4X4]

Put your floor jack on the lower control arm to put pressure on the ball joint stud in the taper, and that nut is what they are using, tighten it to torque specs, the old nut is called a Castle nut.