"Jeez" is not bad. It is a shortened form of "Jesus" -- which can be offensive to some people if used as an oath -- but "jeez" is a version of "Jesus" that is made to be inoffensive. Similar example: "darn" is a version of "damn" meant to be inoffensive -- the type of thing you can say around children. "Jizz" is pronounced differently -- short "i" sound instead of long "e" sound -- and has a totally different meaning. It is slang for semen. It is not very polite slang. You would not want to use this word in most conversation.