The story
From Latin: nūlla (feminine form of nūllus, “not any”) + poena (“penalty”, “punishment”) + sine (“without”) + lēge (ablative singular form of lēx, “a law”) = “no punishment without a law”.
Etymology from Wiktionary, CC BY-SA 4.0.
nulla pœna sine lege
Today it means: The principle that a person shall receive no punishment unless he has committed an offence as explicitly defined in a law. Full definition →