Mark Forsyth (The Inky Fool), The Etymologicon: A Circular Stroll Through the Hidden Connections of the English Language (via wordsspentinvain)
Shit can be traced back to the Old English verb scitan (which meant exactly what it does today), and further back to Proto-Germanic skit (the Germans still say scheisse), and all the way back to the Proto-Indo-European word (c. 4000 BC) skhei, which meant to separate or divide, presumably on the basis that you separated yourself from your faeces. Shed (as in shed your skin) comes from the same root, and so does schism.
An odd little aspect of this etymology is that when Proto-Indo-European arrived in the Italian peninsula they used skhei to mean separate or distinguish. If you could tell two things apart then you knew them, and so the Latin word for know became scire. From that you got the Latin word scientia, which meant knowledge, and from that we got the word science This means that science is, etymologically, shit. It also means that knowing your shit, etymologically, means that you’re good at physics and chemistry.