Untruth comes from un- ('not') plus truth, from Old English trēowþ ('faithfulness, fidelity'), from trēowe ('faithful, true'). In current use it is a noun meaning "a statement that is not true; a lie or falsehood". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The prefix un- is negative or privative, marking the absence, denial, or opposite of the base idea. The surface form is compositional: its joined elements remain visible in the modern word. The historical chain includes Old English un- and Old English trēowþ. Related English forms include truth and true. Derived forms include untruthful and untruthfully. As a result, the modern word is best read as a negative, privative, or reversative formation built on a historically older base.