The Greek adjective akatakritos (ἀκατάκριτος) means uncondemned or not yet tried and sentenced — particularly referring to a Roman citizen who had not received a formal legal hearing. Flogging such a person was a serious violation of Roman law.
Paul's appeal to his Roman citizenship shows the wise use of legitimate earthly authority to advance the gospel. More deeply, in Christ there is no condemnation (Romans 8:1) — not on legal technicality, but because Christ bore the sentence for us. Believers stand uncondemned before God through the substitutionary work of Christ.