The Collaborative International Dictionary
Post-
Post- \Post-\ (p[=o]st). [L. post behind, after; cf. Skr. pa[,c]c[=a]behind, afterwards.] A prefix signifying behind, back, after; as, postcommissure, postdot, postscript.
Douglas Harper's Etymology Dictionary
post-
word-forming element meaning "after," from Latin post "behind, after, afterward," from *pos-ti (cognates: Arcadian pos, Doric poti "toward, to, near, close by;" Old Church Slavonic po "behind, after," pozdu "late;" Lithuanian pas "at, by"), from PIE *apo- (cognates: Greek apo "from," Latin ab "away from" see apo-).
Wiktionary
post-
pre. 1 after, later 2 behind