launa lambið gráa

Etymology
From the saga of Heiðarvíga. In the year 1000, a man named Styr had just killed the farmer Þórhalli from Jörfi who had two children; Áslaug and Gestur. His son Gestur was a weak and backward child, who was not likely to avenge his father's death. Still Styr decided to recompense Gestur for the patricide with a grey. Later however (in the year 1007-1008) Gestur sneaks up to Styr while supping and slashes with an ax full force in Styr's head. He hits behind Styr's right ear so that the brain shows and Gestur speaks; “þar launaði ég þér lambið gráa” which means “with this I've repaid you that grey lamb” and continues to run out the back door. The words themselves come from and  the definite accusative of  and  the weak declension accusative form of.

Idiom

 * 1)  to get revenge on somebody, to take vengeance on somebody