Where does “伯母” come from?

伯母 (Japanese) comes from Japanese 小, from Old Japanese ささ.

伯母 (Japanese): an aunt

Definitions

Step	Language	Word	Meaning
1	Japanese	小	smallness; small size, small; Short for 小の月;...
2	Old Japanese	ささ	—

伯母 — Etymology, Origin & Word History | Etymology Explorer