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.
<a href="https://www.etymonline.com/word/foliation">Etymology of foliation by etymonline</a>
Harper, D. (n.d.). Etymology of foliation. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/foliation