Английский Этимологический словарь - sack (2)

- "plunder," 1549, from M.Fr. sac, in the phrase mettre а sac "put it in a bag," a military leader's command to his troops to plunder a city (parallel to It. sacco, with the same range of meaning), from L. saccus "bag" (see sack (1)). The notion is probably of putting booty in a bag. This is the root of the verb in the U.S. football sense (1969).