Debt is the standard exhibit for silent letters that were never spoken. English took the word about 1300 from Old French dete, and spelled it dette for a century — which is exactly how it sounded then and now. Around 1400 scholars restored a b to it, on the grounds that the French word came from Latin debitum, "thing owed," from debere, "to owe."
The b is therefore not a fossil of English pronunciation. It is an editorial footnote about Latin, inserted into the spelling of a word that had never had that sound in English or in the French it came from. Doubt got the same treatment from dubitare, and receipt from receptum. This is the crucial distinction to hold on to: the silent letters in knight and gnaw record sounds people really made and stopped making, while the silent letter in debt records something a Renaissance scholar wanted you to know.
debt (n.) c. 1300, dette , "anything owed or due from one person to another, a liability or obligation to pay or render something to another," from Old French dete , from Latin debitum "thing owed," neuter past participle of debere "to owe," originally, "keep something away from someone," from de "away" (see de- ) + habere "to have" (from PIE root *ghabh- "to give or receive"). Meaning "state of being under obligation to make payment" is from mid-14c. Restored spelling after c. 1400. In Middle English, debt of the body (mid-14c.) was "that which spouses owe to each other, sexual intercourse." also from c. 1300
debt came in through Latin, and its spelling follows that family's conventions.
What gives it away on the page
| silent letters that spoil the pronunciation | Renaissance scribes inserted them to display a Latin ancestor. See the caveats below — this is the one cue that is often a lie.debt, doubt, receipt, salmon |