As far as I can tell Ars is Art, Tribunus is Tribune and
Angusticlavius is narrow stripe. In Roman society Tribunus
Angusticlavius was a Tribune who wore a uniform/toga with a narrow
purple stripe on the edge denoting his rank. The Technology forum
Ars Technica used to use, and is reintroducing, a ranking system
based on the Roman system. Thus an Ars Technica forum member with a
sufficient ranking would be Ars Tribunus Angusticlavius. That's to
the best of my knowledge.