This is an act of deep distress and grief. This can be written more clearly. Alternate translation: "Jacob was so grieved that he tore his garments" (See: [[rc://en/ta/man/translate/figs-explicit]])
Here "loins" refers to the middle part of the body or the waist. Alternate translation: "put on sackcloth" (See: [[rc://en/ta/man/translate/figs-explicit]])