Yiddish: It will happen when hair grows on the palm of my hand.

It will never happen.   SOURCE: Stutchkoff, Der Oytser fun der Yidisher Shprakh. Variation: replace "hair" with "piroshki" (פּירעשקעס, pireshkes).  Piroshki are small pastries with meat or other fillings.

עס וועט געשען ווען בײַ מיר וועט אויסוואַקסן האָר אויף דער דלאָניע.

Es vet geshen ven bay mir vet oysvaksn hor af der dlonye.

It will happen when hair grows on the palm of my hand.