This week's reading in Old Norse was sections of The Volsung Saga and, in keeping with Signy's rather determined efforts to avenge her father's death, she was egging her brother on. The verb in the Old Norse was eggja, that is "to incite, to urge one on" so the phrase is an old one. I don't know which came first the noun egg or the verb but there are two egg's in Old Norse. One is "egg" of course and the other is "edge". It is the edge that is the relative to the verb because you can whet a knife and that creates an edge. If that does not make sense, think about whetting your appetite. So now you know, egging some one on has nothing to do with eggs and everything to do with sharpening your knife to avenge your father. No yolking.