Definition of f-hour:
(n) :
(printing, abbreviation) Folio, paper and book size (10"-12.5" x 15"-20")
(n) :
An academic grade, specifically:
(n) :
A failing grade, with the next best grade being either a D or an E (some institutions issue Es instead of Fs).
(n) :
(UK) A grade lower than E but higher than G.
(n) :
A grade of black-lead pencil (originally an abbreviation of fine).
(n) :
In the American Library Association, a folio-sized book, over 30 cm in height.