Can mean small fish in Japanese, but also used to describe a person who can't seem to do anything right. If speaking from a company hierarchy point of view, zako's would be placed at the bottom of the list. If such an extreme example of "zako" is encountered, they would usually be the ones cleaning toilets. See: Worthless.
Man, that grilled zako tasted like ass. I think it was because it was some zako making it while he was cleaning the company's toilet room.