Source : Webster's Revised Unabridged Dictionary (1913)

Ex post facto \Ex" post` fac"to\, or Ex postfacto \Ex"
post`fac"to\ ([e^]ks" p[=o]st" f[a^]k"t[-o]). [L., from what is
   done afterwards.] (Law)
   From or by an after act, or thing done afterward; in
   consequence of a subsequent act; retrospective.