From ander(“second”) and halb(“half”), meaning something like 'half way to the second number'. This word is the single surviving form of an older system to describe fractions, which counted from the higher ordinal number rather than the lower cardinal. Compare Middle Low Germananderthalf. The word ander has since lost the meaning 'second', being used solely as 'other' in modern German.
(most of Germany, some of Austria) IPA(key):/andərthalb/, [ˈändɐtʰˈhälpʰ], [ˈandɐtʰˈhalpʰ]
(Switzerland, some of Austria and southern Germany) IPA(key):/andərthalb/, [ˈändərtˈhälp], [ˈɑndərtˈhɑlp]