Nec semel inrisus triviis
And he who has once been fooled (tr. Henry Rushton Fairclough)
Hoc exemplo significat numquam postea crediturum quem esse etiam vera dicenti ei, qui falsis rebus omnia fidei argumenta consumpserit. hinc etiam proverbium natum est “qui semel scurra, numquam pater familias.”
(Porphyrius, Comm. in Hor. Epist. 1.17.58)
By this example [Horace] shows that one will never be believed in the future, even if he is telling the truth, if he has wasted his credibility in telling lies. Hence the proverb has arisen: “Once a scurra*, never a pater familias.”
* A scurra is a fashionable city idler, a man-about-town.
(tr. Christopher Francese, with his note)