An Etymological Dictionary of the German Language/blähen
blähen, vb., ‘to inflate,’ from the equiv. MidHG. blœjen, OHG. blâjan, wk. vb. (the OHG. word also means ‘to blow’); comp. AS. blâwan, E. to blow. The Teut. root blâ (blê) agrees partly with Lat. 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.