Where does “小臼歯” come from?

小臼歯 (Japanese) comes from Japanese 臼歯, from Japanese 歯, from Middle Chinese 齒.

小臼歯 (Japanese): premolar

Definitions

小臼歯 traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Japanese	臼歯	molar
2	Japanese	歯	tooth; age
3	Middle Chinese	齒	—

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