(Sometimes shortened to just "pig".) A rectangular solid made of pig iron, steel, or lead; measuring (depending on manufacturer) 4"-6" wide x 12"-16" long x 1/2"-2" thick used on the arbor of a counterweight fly system or on the stage deck as ballast.
http://theatresafetyblog.blogspot.com/2011_04_01_archive.html
http://www.britannica.com/EBchecked/media/118101/Pig-iron-weights
http://theatresafetyblog.blogspot.com/2011_04_01_archive.html
http://www.britannica.com/EBchecked/media/118101/Pig-iron-weights