Where does “Septimus” come from?

Septimus comes from Latin septimus, the ordinal form of Latin septem (seven), derived from Proto-Indo-European septḿ̥ with the ordinal suffix -ós.

Ancestry of “Septimus”, step by step

Step	Language	Word	Meaning
1	Latin	septimus	seventh
2	Latin	Septem	—
3	Proto-Italic	septem	seven
4	Proto-Indo-European	septḿ̥	seven

Septimus — Etymology, Origin & Word History | Etymology Explorer