Where does “שבתאי” come from?

שבתאי (Hebrew) comes from Hebrew שבת, from Akkadian šapattum — 15.

Ancestry of “שבתאי”, step by step

Step	Language	Word	Meaning
1	Hebrew	שבת	Shabbat, Sabbath; weekly day of rest; Saturday;...
2	Akkadian	šapattum	15