"preceding and leading up to something more important," 1660s, from French préliminaire and directly from Medieval Latin praeliminaris, from Latin prae "before" (see pre-) + limen (genitive liminis) "threshold" (see limit (n.)). A word that arose in reference to painstaking diplomatic negotiations to end the Thirty Years' War. Earliest attested form in English is preliminaries (n.) "things which lead up to or introduce following matter or events," 1650s.
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<a href="https://www.etymonline.com/word/prelim">Etymology of prelim by etymonline</a>
Harper, D. (n.d.). Etymology of prelim. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/prelim