An Etymological Dictionary of the German Language/Annotated/blähen
blähen, verb, ‘to inflate,’ from the equivalent Middle High German blœjen, Old High German blâjan, weak verb (the Old High German word also means ‘to blow’); compare Anglo-Saxon blâwan, English to blow. The Teutonic root blâ (blê) agrees partly with Latin flare (Aryan root bhlâ); blasen, Blatt, and Blatter are also closely related to it. Blasen especially seems to have arisen from the shorter root, also preserved in Blatter, by adding s to the stem of the present.