mid-15c., from Late Latin folio "leaf or sheet of paper," from Latin folio, ablative of folium "leaf" (source also of Italian foglia, French feuille, Spanish hoja), from PIE *bhol-yo- "leaf" (source also of Greek phyllon "leaf," Gaelic bile "leaflet, blossom"), suffixed form of root *bhel- (3) "to thrive, bloom." Ablative of location, because this was used in page references. Meaning "volume of the largest size" first attested 1620s.
*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."
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.