The word incarnation was first used in the 1297 Chronicle of Robert of Gloucester, when it was spelled incarnacion. Other forms since then have included incarnacioun, incarnacione, and incarnacyon; incarnation was standardized by the end of the sixteenth century, after which it has steadily trended upwards in usage. The term comes from Old French incarnacion, which referred specifically to the Incarnation of Jesus (all future definitions evolved from that). That traces to Latin incarnare, meaning "to make into flesh" - a parallel may be drawn to the phrase "in the flesh". Here we may separate the prefix in- ("in") and find the root caro, meaning "flesh". Through Proto-Italic, that derives from the Proto-Indo-European reconstruction ker, or "army". Don't even ask.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.