Foolhardy means recklessly bold or rash. Unwise means lacking good judgment. Both imply poor judgment in action, so they are synonyms.
Foretaste means a preview or sample of what is to come. Anticipation means looking forward to something before it happens. Both express expectation of the future, so they are synonyms.
Forfeit means to lose something as a consequence of wrongdoing. Lose means to be deprived of something. Both involve giving up something, so they are synonyms.
Formidable means inspiring fear or respect due to power or difficulty. Impenetrable means impossible to pass through or overcome. Both suggest something difficult to face, so they are synonyms.
Fortitude means courage in facing difficulties. Braveness means the quality of showing courage. Both relate to strength of character, making them synonyms.
Fortuitous means happening by chance, often positively. Accidental means occurring unexpectedly, without design. Both refer to chance happenings, so they are synonyms.
Fossil means preserved remains or traces of prehistoric life. Prehistoric remnant of an animal or plant is its direct definition. Both are identical in meaning, so they are synonyms.
Foster means to encourage or bring up. Raise means to bring up or nurture. Both express nurturing or developing, hence synonyms.
Founder (as a verb) means to sink or fail. To sink means to go down below the surface, often into water. Both imply going down or collapsing, hence synonyms.
Franchise means official authorization to operate a business or service. License means legal permission to do something. Both refer to official permission, so they are synonyms.