The only free and comprehensive online etymological dictionary of the Spanish language
ferrocarril m. (Noun) "railway"

19th cent. From ferro- "iron" and carril. A calque of English "iron railway," a name used by William Jessop for his horse-drawn plateway in England.
fértil (Adjective) "fertile"

15th cent. From Latin fertilis 'id.,' from ferre "to produce," "to carry." From Proto-Italic *fer-e/o- 'id.' From Proto-Indo-European *bher-e/o- 'id.' A thematic present from the root *bher- "to bear."

Romance: Western Vulgar Latin: Asturian fértil, Portuguese fértil, Galician fértil, Catalan fèrtil, French fertile, Italian fertile

Italic: Oscan ferríns "were to carry," Umbrian fertu "he carried," Marrucinian ferret "he carries," Volscan ferom "to carry"

Indo-European: Celtic: Old Irish beirid "to carry," Middle Welsh beryt "to flow," Middle Breton beraff 'id.,' Cornish kemmeres "to recieve;" Germanic: Gothic bairan "to bear," Old Norse bera 'id.,' Old Saxon beran 'id.,' Old High German beran 'id.,' Old English beran 'id.' (English to bear); Balto-Slavic: Old Church Slavonic bьrati "to gather," Russian brat' "to take," Lithuanian ber̃ti "to scatter;" Albanian: bie "to carry;" Hellenic: Ancient Greek φέρειν (phérein) "to bear;" Phrygian: αβ-βερετ (ab-beret) "to bear;" Armenian: berem "to bear;" Indo-Iranian: Sanskrit bhára- "to bear," Avestan bara- 'id.;' Tocharian: A paräṃ "to bear," B pärtä 'id.'
fiar (Verb) "to guarantee"

12th cent. From Vulgar Latin *fidare "to trust," from Latin fidere 'id.' From Proto-Italic *feiþe- 'id.' From Proto-Indo-European *bhei̯dh-e/o- 'id.'

Romance: Western Vulgar Latin: Portuguese fender, Catalan fendre, French fendre, Italian fendere

Italic: Umbrian fise (deity) "Trust"

Indo-European: Balto-Slavic: Old Church Slavonic běda "distress," Russian bedá "trouble," Czech bída "poverty," Bulgarian bedá "misery;" Albanian: "oath;" Hellenic: Ancient Greek πείθομαι (peíthomai) "to be convinced"
-fico, -ficio, -ficium (Suffix) "maker"

From Latin -ficus and -ficium respectively. Derived from facere "to make" (see hacer).
fiesta f. (Noun) "party," "feast"

13th cent. From Latin festa 'id.' Originally the Latin word of choice was in the masculine, festus. The modern feminine fiesta derives from a longer Roman phrase festa dies "festival days," reinterpreted as a feminine singular noun. From Proto-Italic *fēs-to "holiday." From Proto-Indo-European *dheh1-s-to- "holy." From *dheh1-s- "religious gift." Perhaps from a root *dheh1- "to gift (in a religious context)."

As a surname Fiestas, it was created in reference to Catholic holy days.

Romance: Western Vulgar Latin: Portuguese festo, Italian festa

Italic: Oscan fiísíais "at/by the holidays"

Indo-European: Indo-Iranian: Sanskrit dhiṣā "impetuously," Avestan dāh- "gift"
fin m. (Noun) "end"

12th cent. From Latin finis 'id.' Possibly from Proto-Italic *fīni- 'id.' Of an unclear etymology.

Feminine la fin is a pre-20th cent. poetic form. Also the origin of Fines, a town in Granada, so-called in reference to the town being along limits of the region's confines.

Romance: Western Vulgar Latin: Asturian fin, Portuguese fim, Galician fin, Catalan fi, French fin, Italian fine; Sardinian: fine
final m. (Adjective, Noun) "final," finale;" "ending"

15th cent. From Latin finalis 'id.,' from finis "end" (see fin).

Romance: Western Vulgar Latin: Italian finale
finalmente (Adverb) "finally"

From final and -mente, an adverb-forming suffix.
firme (Adjective) "firm"

12th cent. From Vulgar Latin firmis 'id.,' from Latin firmus "strong." From Proto-Italic *fermo- 'id.' From Proto-Indo-European *dher-mo- "hold." From the root *dher- "to hold."

Romance: Western Vulgar Latin: Portuguese firme, Catalan ferm, French ferme, Italian fermo; Eastern Vulgar Latin: Romanian ferm; Sardinian: fírmu

Indo-European: Indo-Iranian: Sanskrit dhárman- "support," "hold," "law"
flor f. (Noun) "flower"

10th cent. Borrowed from Old French or Old Occitan flor 'id.' From Latin florem, accusative of flos 'id.' From Proto-Italic *flōs 'id.' From Proto-Indo-European *bhleh3- 'id.'

Also the origin of numerous names and surnames: Flores, de la Flor, Flora, Floren, Floria, Florida, Florido, Florentín, Florentino, Florindo, Florencia, Florensa, Florenza, Florencio, Florenciano, Florenzano, Florián, Floriano, Floresca, Florita, to name a few.

Romance: Western Vulgar Latin: Asturian flor, Portuguese flor, Galician chor, Catalan flor, French fleur, Italian fiore; Eastern Vulgar Latin: Aromanian floari, Romanian floare; Sardinian: fiòre

Italic: Vestinian flusare (month name "of Flusa" (goddess of flowers), Oscan fluusaí "to Flusa"

Indo-European: Celtic: Old Irish bláth "flower," Welsh blawd 'id.,' Middle Breton blezu 'id.,' Old Cornish blodon 'id.;' Germanic: Gothic bloma "flower," Old Norse blómi 'id.,' Old Saxon blōmo 'id.' Old English blōwan "to bloom"

The word is assumed to be a loan from a Gallo Romance language on the basis that Latin florem should have yielded **llor. It is interesting that so elementary a lexical item was borrowed, but because other gardening words in Spanish were borrowed (e.g., jardín laurel, clavel), perhaps flor was as well.