Short for CK underwear. The full name of CK is CalvinKlein, which is an American fashion brand. When people mention UB 1 in the CK bag, they mean the abbreviation of CK underwear. This is because CK bag refers to the bag products under CK brand, and UB 1 is the abbreviation of CK underwear, so UB 1 in CK bag is the abbreviation of CK underwear.