Unvirtuously comes from un- ('not') plus virtuously, from virtuous, from Old French vertuos, from Latin virtus ('moral excellence, manliness'), from vir ('man'). In current use it is an adverb meaning "in a way lacking moral goodness or virtue". 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-, Latin virtus, and Latin vir. Related English forms include virtue, virtuous, and unvirtuous. As a result, the modern word is best read as a negative, privative, or reversative formation built on a historically older base.