If it is what I think it is, it simply means cranking two nuts against one another to prevent them from moving. Turnbuckles often use a jam nut to lock them in place after adjustment.
I agree with these definitions in general, however, for a pipe hanger application it may just refer using a nut on both sides of the hole that the threaded rod of the hanger passes through, rather than two nuts on one side.
Sometimes - double nutting would put one on either side of the rod where it passes through the hanger - to prevent the load from going up or down. They would still be cranked against each other!!
Sorry Ione, I did not check your link. When several people post links it can get a little tedious to check each one. However, your link shows two nuts on the lower side of the hanger hole and one above, with the caution to "double nut to prevent hanger failure".
Now I agree that one on each side is important for that design to work. But I do not see how placing a second nut on the bottom does anything useful.
