Etymology
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leaven (v.)
"excite fermentation in," c. 1400, leueyn, from leaven (n.). Figurative sense "work upon by invisible or powerful influence" is from 1540s. Related: Leavened; leavening.
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leaven (n.)
mid-14c., "substance added to dough to produce fermentation," from Old French levain "leaven, sourdough" (12c.), from Latin levamen, which in literary use meant "alleviation, mitigation," but in Vulgar Latin it had a literal sense of "means of lifting, something that raises." It is from levare "to raise" (from PIE root *legwh- "not heavy, having little weight"). Figurative use is from late 14c., "[c]hiefly with allusion to certain passages of the gospels" [OED]. Related: Leavenous.
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unleavened (adj.)
1520s, from un- (1) "not" + past participle of leaven (v.).
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*legwh- 
Proto-Indo-European root meaning "not heavy, having little weight."

It forms all or part of: alleviate; alleviation; alto-rilievo; carnival; elevate; elevation; elevator; leaven; legerdemain; leprechaun; Levant; levator; levee; lever; levity; levy (v.) "to raise or collect;" light (adj.1) "not heavy, having little weight;" lighter (n.1) "type of barge used in unloading;" lung; relevance; relevant; releve; relief; relieve.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit laghuh "quick, small;" Greek elakhys "small," elaphros "light;" Latin levare "to raise," levis "light in weight, not heavy;" Old Church Slavonic liguku, Russian lëgkij, Polish lekki, Lithuanian lengvas "light in weight;" Old Irish lu "small," laigiu "smaller, worse;" Gothic leihts, Old English leoht "not heavy, light in weight."
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ferment (v.)
late 14c. (intransitive), from Old French fermenter (13c.) and directly from Latin fermentare "to leaven, cause to rise or ferment," from fermentum "substance causing fermentation, leaven, drink made of fermented barley," perhaps contracted from *fervimentum, from root of fervere "to boil, seethe" (from PIE root *bhreu- "to boil, bubble, effervesce, burn"). Transitive use from 1670s. Figurative use from 1650s. Related: Fermented; fermenting.
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sourdough (n.)
early 14c., "leavened bread," also "leaven" (late 14c.), from sour (adj.) + dough. Meaning "fermented dough" is from 1868. The meaning "Arctic prospector or pioneer" is from 1898 Yukon gold rush, from the practice of saving a lump of fermented dough as leaven for raising bread baked during the winter.
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ferment (n.)

early 15c., from Old French ferment (14c.), from Latin fermentum "leaven, yeast; drink made of fermented barley;" figuratively "anger, passion" (see ferment (v.)). Figurative sense of "anger, passion, commotion" in English is from 1670s.

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barm (n.)
Old English beorma "yeast, leaven," also "head of a beer," from Proto-Germanic *bhermen- "yeast" (source also of Dutch berm, Middle Low German barm), from suffixed form of PIE root *bhreu- "to boil, bubble, effervesce, burn."
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enzyme (n.)

1881, as a biochemical term, from German Enzym, coined 1878 by German physiologist Wilhelm Kühne (1837-1900), from Modern Greek enzymos "leavened," from en "in" (see en- (2)) + zymē "leaven" (see zymurgy). Related: Enzymotic.

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kvass (n.)

Russian fermented drink made from rye or barley, 1550s, from Russian kvas, from Old East Slavic kvasŭ, from Proto-Slavic *kvasu "leaven, fermented drink," said to be cognate with Latin caseus "cheese" (see cheese (n.1)).

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