The gaming cube, singular of “dice”. “Cast” here means thrown, not formed from liquid hardening in a mold.
It’s a translation of iacta alea est, a phrase attributed to Julius Caesar upon the crossing of the Rubicon.
Lewis and Short, citing Casaubon and Ruhnk, suggest that the text of Suetonius should read Jacta alea esto, which they translate as “Let the die be cast!”, or “Let the game be ventured!”. This matches Plutarch’s third-person aorist imperative ἀνερρίφθω κύβος (anerrhiphtho kybos).
(When I see “this old thread” I think of, say, 2004, not last May.)