Where does “publica” come from?

publica (Latin) comes from Latin pūblicus, from Old Latin poplicus, from Latin pōpulus, from Proto-Italic poplos — army.

Ancestry of “publica”, step by step

Step	Language	Word	Meaning
1	Latin	pūblicus	—
2	Old Latin	poplicus	Alternative form of publicus
3	Latin	pōpulus	—
4	Proto-Italic	poplos	army

publica — Etymology, Origin & Word History | Etymology Explorer