Untruthful comes from untruth plus -ful, ultimately from un- ('not') plus Old English trēowþ ('truth, fidelity'). In current use it is an adjective meaning "not telling the truth; dishonest or false". 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 suffix -ful marks possession or fullness of the quality named by the base. 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, truthful, and untruth. Derived forms include untruthfully and untruthfulness. As a result, the modern word is best read as a negative, privative, or reversative formation built on a historically older base.