Literally, it means 'we cannot', but the literalness is not the point, because these words are a slogan, an incantation, a declaration of fidelity to the truth and to one's principles.
The history of these words comes from the apostles Peter and John, who disobeyed orders from the Sanhedrin - the supreme Jewish religious and judicial institution - forbidding the preaching of Christ's teachings, justifying their actions with the words: for we cannot say what we have seen and heard (non enim possumus quae vidimus et audivimus non loqui). We cannot. Non possumus. We cannot fail to proclaim the truth. We cannot betray our principles. We cannot embezzle.