I know it's not a reliable source by itself, and it lists no citations for the sentence in question, but FWIW, I based my answer on https://en.wikipedia.org/wik

I know it's not a reliable source by itself, and it lists no citations for the sentence in question, but FWIW, I based my answer on https://en.wikipedia.org/wiki/Shabbat#Etymology https://en.wikipedia.org/wiki/Shabbat#Etymology : > Other related words are ... to sheva (שֶׁבַע) meaning seven, as Shabbat is the seventh day of the week ...