underlining I. \ˈ ̷ ̷ ̷ ̷ˌ ̷ ̷ ̷ ̷\noun Etymology:under (III) + lining : lining placed on the under or the inner side of something II. noun Etymology: from gerund of underline (II) : the action of drawing lines underneath in writing or printing or lines so drawn : underscoring < letters … with their underlinings and broken sentences and postscripts — G.H.Genzmer >