Etymology
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foliage (n.)

mid-15c., ffoylage, "representation of leaves or branches" (as an ornamental design). Compare Middle French feuillage, from Old French feuille "leaf, foliage" (from PIE root *bhel- (3) "to thrive, bloom"). The form has altered 17c. by influence of Latin folium or its derivatives in English.

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*bhel- (3)
Proto-Indo-European root meaning "to thrive, bloom," possibly a variant of PIE root *bhel- (2) "to blow, swell."

It forms all or part of: blade; bleed; bless; blood; blow (v.2) "to bloom, blossom;" bloom (n.1) "blossom of a plant;" bloom (n.2) "rough mass of wrought iron;" blossom; cauliflower; chervil; cinquefoil; deflower; defoliation; effloresce; exfoliate; feuilleton; flora; floral; floret; florid; florin; florist; flour; flourish; flower; foil (n.) "very thin sheet of metal;" foliage; folio; folium; gillyflower; Phyllis; phyllo-; portfolio; trefoil.

It is the hypothetical source of/evidence for its existence is provided by: Greek phyllon "leaf;" Latin flos "flower," folio, folium "leaf;" Middle Irish blath, Welsh blawd "blossom, flower;" Gaelic bile "leaflet, blossom;" Old English blowan "to flower, bloom."
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greenery (n.)
"mass of green plants or foliage," 1797, from green (n.) + -ery. From 1836 as "place where plants are reared."
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leaf (v.)
"to turn over (the pages of a book)," 1660s, from leaf (n.). Meaning "put forth leaves or foliage" is from 1610s. Related: Leafed; leaved; leafing.
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frond (n.)
1785, from Latin frons (genitive frondis) "leafy branch, green bough, foliage." Adopted by Linnæus for the leaf-like organs of ferns, palms, etc., as a word distinct from folium. Later given a more precise meaning in botany.
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vert (n.)
mid-15c., "the color green" (especially in heraldry), also "trees and brush bearing green leaves" (in forest law), from Anglo-French and Old French vert "foliage, greenery, green cloth," from Latin viridem, viridis "green" (see verdure).
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defoliation (n.)
Origin and meaning of defoliation

1650s, "loss of leaves," noun of action from past-participle stem of Late Latin defoliare "shed leaves," from de (see de-) + folium "leaf" (from PIE root *bhel- (3) "to thrive, bloom"). Often especially of the fall of leaves in autumn. Meaning "deliberate destruction of foliage" (for military purposes) is from 1964.

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overgrown (adj.)

late 14c., "covered with growth," past-participle adjective from overgrow "to cover, overspread (with foliage);" late 14c., overgrouen, see over- + grown, and compare Old English verb ofergrowan. Meaning "having grown too large, grown beyond the fit or natural size" is attested from late 15c.

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leaf (n.)

Old English leaf "leaf of a plant, foliage; page of a book, sheet of paper," from Proto-Germanic *lauba- (source also of Old Saxon lof, Old Norse lauf, Old Frisian laf, Dutch loof, Old High German loub, German Laub "foliage, leaves," Gothic laufs "leaf, foliage"), perhaps from PIE *leub(h)- "to peel off, strip or break off" ((source also of Old Irish luib, "herb," lub-gort "garden;" Albanian labë "rind, cork;" Lithuanian luba "plank, board;" Russian lob "forehead, brow," Czech leb "skull;" Lithuanian luobas "bast," Latvian luobas "peel," Russian lub "bast;" Old Norse lyf "medicinal herbs," Old English lybb "poison; magic").

Related to lodge and lobby; for another PIE root see folio. Extended late 14c. to very thin sheets of metal (especially gold). Compare Lithuanian lapas "leaf," from a root also in Greek lepos "bark," lepein "to peel off." Also applied to flat and relatively broad surfaces, especially of flexible or mounted attachments; meaning "hinged flap on the side of a table" is from 1550s. To turn over a (new) leaf (1590s; 1570s as turn the leaf) "begin a new and better course of life" is a reference to the book sense. Among insects, leaf-hopper is from 1847; leaf-cutter from 1816.

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cypress (n.)

popular name of a type of evergreen tree noted for its dense, dark foliage and durable, fragrant wood, native to southern Europe and sacred to Pluto, late 12c., from Old French cipres (12c., Modern French cyprès), from Late Latin cypressus, from Latin cupressus, from Greek kyparissos, probably from an unknown pre-Greek Mediterranean language.

Perhaps it is related to Hebrew gopher, name of the tree whose wood was used to make the ark (Genesis vi.14). Extended to similar trees of America, Australia, and Japan. An emblem of mourning for the dead, cypress branches were used at funeral.

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