1660s, "body fluid," from Latin latex (genitive laticis) "liquid, a liquid, fluid," probably from Greek latax "dregs," from PIE root *lat- "wet, moist" (source also of Middle Irish laith "beer," Welsh llaid "mud, mire," Lithuanian latakas "pool, puddle," Old Norse leþja "filth").
From 1835 as "milky liquid from plants." Meaning "water-dispersed polymer particles" (used in rubber goods, paints, etc.) is from 1937. As an adjective by 1954, in place of the classically correct laticiferous.