Is CILLS a Wordle Word?

Yes. CILLS 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 CILLS mean?

(also window sill) A horizontal slat which forms the base of a window.
A horizontal, structural member of a building near ground level on a foundation or pilings or lying on the ground in earth-fast construction and bearing the upright portion of a frame. Also called a ground plate, groundsill, sole, sole-plate, mudsill. An interrupted sill fits between posts instead of being below and supporting the posts in timber framing.
A horizontal layer of igneous rock between older rock beds.

3 more noun definitions

A piece of timber across the bottom of a canal lock for the gates to shut against.
A raised area at the base of the nasal aperture in the skull.“the nasal sill”
The inner edge of the bottom of an embrasure.

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
Unique letters: 4 of 5Repeats L
Common letters: I, L

Hard-mode trap: 19 valid words fit the pattern _ILLS. 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.