mid-14c., "having to do with the past," from Old French preterit "past tense" (13c.) and directly from Latin praeteritum (as in tempus praeteritum "time past"), past participle of praeterire "to go by, go past," from praeter "beyond, before, above, more than" (see prae-) + itum, past participle of ire "to go" (from PIE root *ei- "to go").
Grammar sense is late 14c. The word also was a noun in Middle English meaning "past times" (late 14c.). Related: Preteritive. Preterite-present attested from 1813.