Meanings
historical · humorousin post-WW2 Germany, a written attestation of not having been a collaborator or associated with the Nazi regime, penned by a third party
figurative · humorous · colloquialan exoneration or dismissal of someone being responsible for past harm (the exoneration is often implied to be incorrect or dishonest)
figurative · colloquialcarte blanche, free rein (a dismissal of the possibility of future harm, effectively allowing someone to do as they please)