or Edge-on. A corruption of the Saxon eggian (to incite). The Anglo-Saxon ecg, and Scandinavian eg, means a “sharp point”—hence edge-hog (hedgehog), a hog with sharp points, called in Danish pin-swin (thorny swine), and in French porc-épic, where épic is the Latin spicula (spikes).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Egg-on from Infoplease: