What does Boxing Day have to do with boxing?
"person in charge of money," 1590s, from Middle French caissier "treasurer," from caisse "money box" (see cash (n.)). The immediate source of the English word might be Middle Dutch kassier.
"dismiss," 1590s, from Middle Dutch casseren, kaseeren "to cast off, discharge," from French casser "to discharge, annul," from Late Latin cassare "annul," from Latin cassus "void, empty" (see caste (n.)). Related: Cashiered; cashiering.