ex-

Where it comes from

Etymology tree

Proto-Indo-European *h₁éǵʰ

Proto-Indo-European *-s

Proto-Indo-European *h₁éǵʰs

Proto-Italic *eks

Latin ex

Latin ex-der.

Middle French ex-bor.

Middle English ex-

English ex-

From Middle English, from words borrowed from Middle French; from Latin ex (“out of, from”), from Proto-Indo-European *eǵ-, *eǵs- (“out”), *eǵʰs. Cognate with Ancient Greek ἐξ (ex, “out of, from”), Transalpine Gaulish ex- (“out”), Old Irish ess- (“out”), Old Church Slavonic изъ (izŭ, “out”), Russian из (iz, “from, out of”).

For sense 3 (“former”), compare sense 11 at Latin ex (indicates a change of state, (later) used before terms denoting an office to indicate that one has completed his term of office). It is probably a semantic extension from the sense of "out, away".