The noun betrayal was first used around 1600, but in verb form it was around since the beginning of the 1200s, when it was spelled betrai, betraye, or betraȝe with a yogh. In its earliest days, it was stylized be-trai, be-traye, and be-traȝe. That's because the term is composed of the word be (the same as we know today; through Proto-Germanic bi, this traces to Proto-Indo-European hepi, "at" or "near") and the Old French root traine, which still meant "betray" (apparently the prefixation of be didn't change much). Traine comes from Latin tradere, meaning "to hand over", so a betrayal is tantamount to handing someone over to their enemies. That in turn is a portmanteau of trans, meaning "across", and dare, "to give". Trans derives from Proto-Indo-European terh, meaning "throughout", and dare is from deh, same definition.
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.