prep n.

late 14c., preparacioun, "act of preparing or making ready, preliminary act or operation, a previous setting in order," from Old French preparacion (13c.) and directly from Latin praeparationem (nominative praeparatio) "a making ready," noun of action from past participle stem of praeparare "prepare," from prae "before" (see pre-) + parare "make ready" (from PIE root *pere- (1) "to produce, procure"). Meaning "a substance especially prepared or manufactured" is from 1640s.

"serving to prepare the way for something to follow," early 15c., preparatori, from Late Latin praeparatorius, from Latin praeparatus, past participle of praeparare "make ready beforehand" (see prepare).

Earlier in same sense was preparative (c. 1400). The word was applied by 1822 in the United Kingdom to junior schools in which pupils are "prepared" for a higher school.