Where does the 'J' in SSJ and SSJ2 come from?
It comes from the Japanese original name for Super Saiyan: 超{スーパー}サイヤ人{じん}, pronounced Sūpā Saiya-jin.

You can read more about it at http://dragonball.wikia.com/wiki/Super_Saiyan.

Jin means "people/person." 'Gaikoku-jin' means foreigners.