F91 may refer to: F91 Dudelange, a football club, based in Dudelange in southern Luxembourg Mobile Suit Gundam F91, a 1991 animated film , a 1981 British Royal Navy Type 22 frigate , a British Royal Navy Blackwood class second-rate anti-submarine frigate Republic XF-91 Thunderceptor, a rocket-jet hybrid interceptor Casio F-91W, a common and inexpensive digital watch, claimed by some to be associated with terrorism The McCarthy 91 function, a recursively defined mathematical function and also: Conduct disorders ICD-10 code