/ˈtæsɪt/
[adjective] Understood or agreed upon without being openly said. A tacit agreement is one that everyone accepts without needing to put it into words.
Tacit comes from the Latin word for 'silence' — meaning every quiet nod you give is etymologically a form of saying nothing at all.
The word 'tacit' travels to us almost unchanged from the Latin adjective 'tacitus', meaning 'silent' or 'unspoken'. That Latin root itself came from the verb 'tacere', which meant simply 'to be silent' or 'to hold one's tongue'. It is one of those rare words that arrived in English already fully formed, needing almost no adjustment to fit comfortably into the language.
Latin 'tacere' was a word of some elegance in the ancient world. Roman writers used it with precision to describe meaningful silence — not the silence of ignorance, but the silence of one who knows and chooses not to speak. The great orator Cicero and the historian Tacitus both explored the power of what is left unsaid, and it is no coincidence that the Roman historian's own name, Tacitus, likely derived from this very root, suggesting a man either famously quiet by nature or named after a family that valued restraint.
The word entered English during the early 17th century, carried in on the wave of Latinate vocabulary that flooded the language during the Renaissance and the following century. Educated writers of the time were deeply familiar with classical Latin texts, and they imported 'tacit' directly from written Latin into formal English prose. Its first recorded uses in English appear around 1600 to 1610, where it described laws or agreements that were implied rather than written down — particularly in legal and philosophical writing.
What makes 'tacit' so interesting is that it describes a very human phenomenon: the agreements we make without making them. A tacit understanding between two people is in some ways more powerful than a spoken one, because it implies a shared knowledge so deep that words are unnecessary. This is quite different from simply forgetting to say something — a tacit arrangement is deliberate in its silence. The word captures the intelligence of implication.
The same Latin root 'tacere' gave English several close relatives. 'Taciturn', meaning a person who habitually speaks very little, came into English via Latin 'taciturnus', following the path Latin taciturnus → French taciturne → English taciturn. The noun 'taciturnity' followed the same route. There is also 'reticent', which, while from a different Latin root ('reticere', meaning to keep silent, a compound of 're-' and 'tacere'), shares the same ancient spirit of deliberate quietness. Together, these words form a small family in English dedicated to the art of not speaking.
Today, 'tacit' is used most often in phrases like 'tacit approval' or 'tacit agreement', particularly in politics, business, and law — fields where what is not said can be just as binding as what is. It remains a quietly powerful word, which seems entirely appropriate for something rooted so deeply in the idea of silence.