"Egging someone on"

Explained by Michael Quinion at World Wide Words:

Despite its spelling, it has nothing to do with actual ova. Those involved are not being persuaded into their actions through fear of being pelted with eggs or of being left with egg on their faces, nor are eggs employed in any other way. The source is quite different and its spelling is accidental, the result of orthographical convergence.

The origin is actually the Old Norse eggja, to incite, which is related to Old English ecg, an edge, and to the Middle Low German eggen, to harrow... the word came into English around the year 1200, initially in the sense of provoking or tempting a person. Our modern form isn’t so ancient, but old enough, appearing in the middle of the sixteenth century.

By the time egg somebody on had appeared, the spelling had changed through being influenced by eggs of the consumable sort.

Not particularly important, but posted to amuse my Eggen in-laws, who can now ponder the personalities of their ancestors...