Mid 19th century; earliest use found in ‘George Eliot’ (1819–1880), novelist (real name Mary Ann Evans). From classical Latin post ēventum after the event, originally via German post eventum.
Definition of post eventum in US English:
post eventum
adverbˌpəʊstɪˈvɛntəm
= post factum.
Origin
Mid 19th century; earliest use found in ‘George Eliot’ (1819–1880), novelist (real name Mary Ann Evans). From classical Latin post ēventum after the event, originally via German post eventum.