I always think of the word concubine in historical terms. To me, it brings to mind emperors' harems, but apparently it can also refer to a partner in any sexual relationship where the couple is unmarried. This was the sole definition when the term was borrowed into English in the late thirteenth century from Anglo-Norman, but it has since evolved. That comes from the Latin verb concumbere, which meant "to lie with" and was composed of con-, meaning "with", and cubare, meaning "to lie down". Con-, through Proto-Italic and the earlier Latin word cum, derives from the Proto-Indo-European root kom, which could also mean "next to". Cubare (also the etymon of English cubicle and cubit) traces to a Proto-Indo-European word sounding like kewb or kub, but definition is uncertain.
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.