The concept of a semester was originally used in German universities, and English borrowed the word for them in the year 1826. It originally comes from Latin semestris, which meant "of six months" (the German system actually is six months long, although most American colleges now use about five). Semestris is composed of two other Latin words: sex, meaning "six", and mensis, meaning "month". This is very similar to the word trimester, which literally means "three months" (extant in pregnancy terminology and another academic term type that is not necessarily three months long anymore). Ultimately, sex comes from the Proto-Indo-European root sweks, which also meant "six", and mensis, through a Proto-Indo-European word meaning "moon", likely traces to the reconstruction meh, "to measure".
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.