In Middle English, debt was spelled dette, and in Old French, it was spelled dete. However, when we trace it to Latin, it was debitum, from the verb debere, which had the meaning of "to owe". This is the interesting part: Middle English scholars figured that, based on the Latin spelling, our word for debt was missing a b, so they inserted a b into dette and it evolved into the modern term, which is why there's a silent b in debt today. Other similar words, like subtle and doubt, developed similarly: because of a Latin root with a b in the past, we're now stuck with weird spellings. From debere, we can identify the prefix de-, meaning "from", "of", or "away", and habere, the verb for "to have". Through Proto-Italic, this derives from the Proto-Indo-European reconstruction gehb, meaning either "to give" or "to receive" (so debt can be interpreted as "to give away"). Surprisingly, usage of the word debt seems to have flatlined since the 1800s.
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.