Is SIXES a Wordle Word?

Yes. SIXES is a valid Wordle word.

It is not on the Wordle answer list, so it is unlikely to be a daily answer.

Plurals ending in S are almost never chosen as Wordle answers.

What does SIXES mean?

The digit or figure 6.
(by ellipsis of six o'clock) Rear, behind (rear side of something).“cover my six”
An event whereby a batsman hits a ball which does not bounce before passing over a boundary in the air, resulting in an award of 6 runs for the batting team.

2 more noun definitions

Definition from Wiktionary, available under the CC BY-SA license.

Starter word rating: WeakIn the top 76% of all valid Wordle words for testing common letters.
Vowels: I, E
Unique letters: 4 of 5Repeats S
Common letters: I, E
Rare letters: X

Hard-mode trap: 11 valid words fit the pattern SI_ES. If you reach it, you may need several guesses to find the last letter.

Letter ratings use how often each letter appears in Wordle answers.

Tap its tiles to the colours Wordle showed, and the Wordle Solver lists every word that still fits, with the best next guess.