http://www.thefreedictionary.com/raja
Word History: Rajah is familiar to us from the Sanskrit rj, "king," and mahrj, "great king." The Sanskrit root raj-, "to rule," comes from the Indo-European root *reg-, "to move in a straight line, direct, rule." The same Indo-European root appears in Italic (Latin) and Celtic. Rx means "king" in Latin, coming from *reg-s, whence our regal and, through French, royal. Two of the Gaulish kings familiar to us from Caesar, Dumnorix and Vercingetorix, incorporate the Celtic word rx, "king," in their names. (Rx also forms part of the name of that fictitious, indomitable Gaul Asterix.) Germanic at some time borrowed the Celtic word rx. It appears as reiks, "ruler," in Gothic, as well as in older Germanic names ending in -ric, such as Alaric and Theodoric, the latter of whom has a name that is equivalent to German Dietrich, "people's king." A derivative of Celtic rx, *rg-yo-, meaning "rule, domain," was also borrowed into Germanic, and is the source of German Reich, "rule, empire."