Back Up 4 : To hold one section of an object, such as a pipe or a nut, while another is being screwed into or out of it. A BACK-UP WRENCH refers to any wrench being used to hold the pipe or bolt. (ITOGP)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}