(paper, printing) Abbreviation of folio., page and book size (10"-12.5" x 15"-20").

(informal) Alternative spelling of fo'

(aviation) Initialism of first officer. / Initialism of field officer. / (logic, computer science) Initialism of first-order logic.

(Internet slang, text messaging, vulgar) Initialism of fuck off.

(dialect, African-American Vernacular) Pronunciation spelling of for. / (dialect, African-American Vernacular) Pronunciation spelling of four.

(archaic) Buddha, in the context of Chinese Buddhism.

Abbreviation of folio.

Initialism of freedom of speech.

plural of FO

(pharmacology) Used to form names of phosphorous derivatives used as insecticides, anthelminthics, pesticides, etc.