early 14c., quantite, "amount, magnitude, the being so much in measure or extent," from Old French quantite, cantite (12c., Modern French quantité) and directly from Latin quantitatem (nominative quantitas) "relative greatness or extent," coined as a loan-translation of Greek posotes (from posos "how great? how much?") from Latin quantus "of what size? how much? how great? what amount?," correlative pronominal adjective (from PIE root *kwo-, stem of relative and interrogative pronouns).
From late 14c. as "that which has quantity, a concrete quantity;" from 1610s in the concrete sense of "an object regarded as more or less." In prosody and metrics, "the relative time occupied in uttering a vowel or syllable" (distinguishing it as long or short) by 1560s. Latin quantitatem also is the source of Italian quantita, Spanish cantidad, Danish and Swedish kvantitet, German quantitat.