Most other basic nouns are Germanic in English, but you can just tell off of the ou- that this is French. Indeed, it derives from the word cosin, entering our language around when the Normans invaded. This is from the Latin word consobrinus, which also meant "cousin". However, as we move backward in time, an older form of the word is revealed: consobrinus once had a much more specific definition, that of "the son of your mother's sister". Yes, cousin was once exclusive to the mother's side of the family. Consobrinus is a portmanteau of the prefix con-, "with", and the root sobrinus (the etymon of Spanish sobrina, "niece"), still with the same definition. This is from the even earlier root soror, meaning "sister", which allegedly stems from the Proto-Italic word swezrinos, "of the sister", from Proto-Indo-European swesor, also "sister". Here it gets even more interesting: the PIE word may be a compound of swe, "self", and hesh, "blood", since a sister has the "blood" of your "self". Interesting...
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd