An Etymological Dictionary of the German Language/rüstig

by Friedrich Kluge

rüstig, adj., ‘prepared for action, vigorous, robust,’ from MidHG. rüstec, ‘vigorous, armed,’ OHG. rustîg, ‘prepared, adorned.’ With regard to the evolution of meaning comp. fertig, also entrüstet. OIc. hraustr, ‘brave, competent,’ is more remote.