The word for a thing that is owed debt first appears in English in the 1200s as a borrowing from Old French dete. As a reminder, in the 1200s in England, the peasants spoke English and the nobility spoke French. The King of England did not speak English. Usually, we retain the English words from that period, but sometimes we retain the French word instead, or both words (e.g. English-derived help and French-derived aid, or English-derived lake and French-derived lagoon). In the case of debt, we retained just the French word and lost the Middle English word shild (Old English scyld, German cognate Schuld).

So wait, if we borrowed the word from dete, and spelled it dette as pronounced in the 1300s, where did the silent ‘b’ come from? In the 1400s, literate people in England typically knew Latin and believed English should be more like the ursprache Latin. Well, in Latin, there’s this word dēbitum, meaning debt, that the French word is descended from. So scribes from that period all ended up agreeing to put the vestigial ‘b’ back into the spelling, cursing us to this day.

Dēbitum would of course later be borrowed into English on its own merits as debit, but that wouldn’t happen for several hundred years.