Fortitudine vincimus - By endurance we conquer
Fortius quo fidelius - Strength through loyalty
Fortuna amicos parat, inopia amicos probat - The fortune is preparing friends, the abundance is testing them

Fortuna vitrea est; tum cum splendet frangitur

Fortune is glass; just when it gleams brightest it shatters

Fortuna caeca est - Fortune is blind. (Cicero)
Fortunatus sum! Pila mea de gramine horrido modo in pratum lene recta volvit! - Isn't that lucky! My ball just rolled out of the rough and onto the fairway!
Frangar non flectar - I am broken, I am not deflected
Frater, ave atque vale - Brother, hello and good-bye. (Catullus)
Fronti nulla fides - No reliance can be placed on appearance. (don't judge a book by its cover)
Frustra laborant quotquot se calculationibus fatigant pro inventione quadraturae circuli - Futile is the labor of those who fatigue themselves with calculations to square the circle. (Michael Stifel, 1544)