An Etymological Dictionary of the German Language/Annotated/billig
billig, adjective, adverb, ‘reasonable (-ably), cheap (-ly),’ for an earlier billich, used even in the last century, from Middle High German billîch, Old High German (recorded since Williram) billîch (adverb Middle High German billîche, Old High German billîhho). ‘conformable, becoming’; cognate with Anglo-Saxon bilewit, Middle English bilewit, ‘simple, innocent.’ It has been said, without sufficient reason, that this class was borrowed from Keltic. Compare other cognates under Weichbild, Unbill.