An Etymological Dictionary of the German Language/reich
reich, adj., ‘rich, copious, abounding,’ from MidHG. rîhhi, OHG. rîhhi, adj., ‘mighty, rich, splendid’; corresponding to OSax. rîki, ‘mighty, powerful,’ AS. rîce, ‘mighty, powerful,’ E. rich, Goth. reiks, ‘mighty, distinguished.’ From the G. adj. the equiv. Rom. cognates are derived; comp. Ital. ricco, Fr. riche, ‘rich.’ The comon Teut. adj. is a derivative of the root *rîk, ‘king,’ discussed under the preceding word, hence ‘mighty’ is the earlier meaning of the cognates; ‘royal’ (Lat. régius) is the orig. sense.