An Etymological Dictionary of the German Language/zwölf
zwölf, num., ‘twelve,’ from the equiv. MidHG. zwęlf, zwęlif, OHG. zwęlif. A common Teut. num.; corresponding to OSax. twęlif, Du. twaalf, AS. twęlf, E. twelve, Goth. twalif, OIc. tolf, Dan. tolv, Swed. tolf. It is a compound of Teut. twa- (HG. zwei), with the component -lif, which appears also in elf (Goth. ain-lif). In the allied Aryan languages a corresponding form occurs only in Lith. twylika, ‘twelve,’ vënolika, ‘eleven’). For the signification of the second component, Teut. -lif, Lith. -lika, see elf.