Where does “-usculus” come from?

-usculus (Latin) comes from Latin -culus, from Proto-Italic *-kelos — Diminutive suffix.

Ancestry of “-usculus”, step by step

Step	Language	Word	Meaning
1	Latin	-culus	Alternative form of -ulus; added to a noun to...
2	Proto-Italic	*-kelos	Diminutive suffix