adj

• (chiefly figurative) Acting, happening, or moving quickly; fast, rapid, swift; also, abrupt, sudden, unexpected.
  "[W]ere the Aer copiouſly mixed vvith the Sap here, as in the Pith, Fruit, and other Parenchymous Parts; it vvould give ſo quick a Ferment to the Sap, as to dilate and amplify the Bladders of the Seed, beyond its preſent compact and durable Texture; and ſo expoſe it, either to a precipitant Grovvth, or ſudden Rot."
• (chemistry) That causes precipitation (“formation of a heavier solid in a lighter liquid as a result of a chemical reaction”).

adv

• (archaic) Synonym of precipitantly (“in a precipitant or headlong manner; with foolish or rash haste”).
  "The Hoary Froſts, and Northern Blaſts take care / Thy muddy Bev'rage to ſerene, and drive / Præcipitant the baſer, ropy Lees."

noun

• (chemistry) A substance that, when added to a solution, causes a dissolved substance to form a precipitate.
  "[B]y putting ſome of the arſenical liquor into a ſtrong ſolution of common ſublimate made in fair vvater […] vve had a copious precipitate, ſuch as might have been expected from an alcaline precipitant; […]"