445. Feasible, Possible

Feasible: Capable of being done
Possible: Able to happen

446. Fervor, Passion

Fervor: Intense feeling
Passion: Strong emotion

447. Fidelity, Loyalty

Fidelity: Faithfulness
Loyalty: Devotion

448. Filch, Steal

Filch: Steal secretly
Steal: Take unlawfully

449. Flagrant, Obvious

Obvious: Easily seen

450. Flout, Disregard

Flout: Show contempt
Disregard: Ignore

451. Forbear, Refrain

Forbear: Hold back

452. Forlorn, Desolate

Forlorn: Lonely and sad
Desolate: Empty and bleak

453. Forsake, Abandon

Forsake: Leave behind

454. Frivolous, Trivial

Frivolous: Not serious

455. Frugal, Economical

Economical: Efficient in spending

456. Futile, Useless

Futile: Ineffective
Useless: Serving no purpose