An Etymological Dictionary of the German Language/zittern
zittern, vb., ‘to tremble, shake, quiver,’ from the equiv. MidHG. zitern, zittern, OHG. zittarôn, wk. vb.; corresponding to OIc. titra, ‘to twinkle, wink, tremble’ (old tr remains unpermutated in HG.; comp. bitter, Splitter, and treu). Zittern is one of the few Teut. vbs. which have a reduplicated present (see beben). From the implied primit. Teut. *ti-trô-mi the transition to the weak ô conjugation is easily understood, just as the change of Teut. *rî-rai-mi, ‘I tremble’ (comp. Goth. reiran, ‘to shake,’ from an Aryan root rai-), to the similarly sounding weak ai conjugation. In the non-Teut. languages no cognates of zittern have been found (Aryan root drā̆?). The G. word was adopted by Dan.; comp. Dan. zittre, ‘to shake.’