also postgraduate, 1858, in reference to a course of study pursued after graduation, originally American English, from post- + graduate (adj.). As a noun, "one studying after graduation," attested from 1890. Abbreviation post-grad is recorded from 1950.
posterior
posteriority
posterity
postern
post-glacial
post-graduate
post-haste
post-hole
post-horse
posthumous
postillon