An Etymological Dictionary of the German Language/sollen
sollen, vb., ‘to owe, be in duty bond, be said to,’ from MidHG. soln (scholn), OHG. solan (scolan), pret. pres., ‘to owe, be obliged, be allowed, become, be indebted, be fitting.’ The corresponding abstract Schuld, f., ‘debt, guilt,’ proves, like Goth. skulan, ‘to be indebted, be bond to pay,’ that skal, ‘to owe,’ is the root (the loss of the guttural, by which the 1st per. skal became sol in OHG. and MidHG., is surprising). From this root a pret. pres. common to Teut. was formed, which assumed the function of an auxiliary vb.; comp. E. shall and Du. zal. For further details see grammars.