mid-15c., from Late Latin folio "leaf or sheet of paper," from Latin folio, ablative of folium "leaf," from PIE *bhol-yo- "leaf" (cognates: Greek phyllon "leaf," Gaelic bile "leaflet, blossom"), suffixed form of root *bhel- (3) "to thrive, bloom," which is possibly from *bhel- (2) "to blow, inflate, swell" (see bole). Ablative of location, because this was used in page references. Meaning "volume of the largest size" first attested 1620s.