Unlatch comes from the prefix un- 'reverse, undo' plus latch 'to fasten with a latch,' meaning 'to release a latch; to unfasten.'. In current use it is a verb meaning "to unfasten or release something secured with a latch". 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 reversative here, marking an undoing or release rather than mere negation. The surface form is compositional: its joined elements remain visible in the modern word. As a result, the modern word is best read as a negative, privative, or reversative formation built on a historically older base.