The only free and comprehensive online etymological dictionary of the Spanish language
fui, fue, fui-, fuer-, fues- (Preterite, imperfect, and future conjugation forms in verb ser.)

From respective Latin forms fui, fue, fui-, fuer-, fues- with the meaning of "become."

From Proto-Italic stem *fu- "to become." From Proto-Indo-European *bhh2u̯- 'id.'

Present tense and future indicative forms derive from Latin sedere "to sit." The verb collapsed sedere and essere "to be" with fui into a single new verb meaning "to be."
fumar (Verb) "to smoke"

Borrowed from Latin fumare 'id.,' a verb derived from the noun fumus "smoke" (see humo).

Romance: Western Vulgar Latin: Asturian fumar, Portuguese fumar, Catalan fumar, French fumer, Italian fumare; Eastern Vulgar Latin: Aromanian fumedz, Romanian fuma; Sardinian fumare
función f. (Noun) "function"

17th cent. From Latin functio 'id.' A noun formed from functus, the past participle of fungi "to perform." From Proto-Italic *fung-e/o- 'id.' From Proto-Indo-European *bhu̯-n-g- "to be used."

Romance: Western Vulgar Latin: Portuguese função, Catalan funció, French fonction, Italian funzione; Eastern Vulgar Latin: Romanian funcțiune

Indo-European: Celtic: Old Irish -bong "breaks," Middle Welsh diwung "to attack;" Germanic: Gothic biugan "to bow," Old Norse beygja "to bend," Old High German bougian 'id.,' Old Saxon bōgian 'id.,' Old English būgan "to bend" (English to bow); Balto-Slavic: Russian bgat' "to bend," Lithuanian gùbti "hill" (metathesis of *bùgti), Latvian baũgursArmenian: bowcanem "to feed;" Indo-Iranian: Sanskrit bhuñjaté "to benefit," Old Avestan būj- "atonement"

"Lat. fungor must be linked with Skt. bhuṅkte, present middle, a nasal form (cf. bhuj-), the primary sense of which is “enjoy”; but at an early date it became specified in the sense of “enjoying food, consume.” This links up with the Armenian bucanem ‘to nourish, bring up’." ~ E. Benveniste, Indo-European Language and Society (1973)
funcionar (Verb) "to function"

19th cent. A verb formed from función.
fundamento m. (Noun) "foundation"

13th cent. From Latin fundamentum 'id.,' from the verb fundare "to found" (see fundar).
fundar (Verb) "to found"

14th cent. From Latin fundare "to found," derived from fundus "bottom" (see hondo.

Romance: Western Vulgar Latin: Asturian fundir, Portuguese fundir, Galician fundir, Catalan fondre, French fondre, Italian fondere
futuro m. (Adjective, Noun) "future"

15th cent. From the Latin future participle futurus "about to be." From Proto-Indo-European *bhh2u̯-tó- "about to become." See more information under fui.

Romance: Western Vulgar Latin: Portuguese futuro, Catalan futur, French futur, Italian futuro