An Etymological Dictionary of the German Language/desto

by Friedrich Kluge

desto, adv., ‘so much the,’ from the equiv. MidHG. dëste, dëst, late OHG. dësde; in an earlier form two words, dës diu (dës, gen., diu, instr. of the art.); the Goth. word was simply þê (instr. of the art.); thus, too, AS. þŷ before comparatives, E. the (the more, desto mehr).