Where does “Cassian” come from?

Cassian comes from Latin Cassianus, a Roman cognomen of uncertain ultimate origin and meaning.

Ancestry of “Cassian”, step by step

Step	Language	Word	Meaning
1	Latin	Cassianus	—
2	Latin	Cassius	—
3	Latin	cassus	hollow, empty, devoid of something; lacking;...
4	Latin	cadō	—
5	Proto-Italic	kadō	fall, sink
6	Proto-Indo-European	ḱad-	to fall