Where does “arbitra” come from?

arbitra (Latin) comes from Latin -a, from Proto-Italic -ā, from Proto-Indo-European -éh₂.

arbitra (Latin): second-person singular present active imperative...

Definitions

Step	Language	Word	Meaning
1	Latin	-a	suffixed to the roots of verbs, forms agent...
2	Proto-Italic	-ā	Forms masculine agent nouns from prefixed or compounded bases
3	Proto-Indo-European	-éh₂	—