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

16th cent. borrowing from Latin baculum 'id.' Of unknown origin. Borrowed from a non-Indo-European language with a root *bak(k)- "stick," "staff."

Romance: Western Vulgar Latin: Portuguese báculo, Galician báculo, French bâcle, Italian bacolo

Celtic: Goidelic: Old Irish bacc "crooked staff," "hook" (bakko-); Brythonic: Old Welsh bach "hook," Old Breton bah 'id.,' Old Cornish bagh 'id.'

Germanic: West Germanic *pagila- bak-: Middle Dutch pegel "peg," Old English pægel "wine jug"

Balto-Slavic: Baltic: Lithuanian bàkstelėti? "to thrust," Latvian bakstît? "to poke"

Hellenic: Ancient Greek βάκτρον (báktron) "stick" (bak-tro-), βακτηρία (baktería) "staff," "scepter" (bak-tér-)

"A more widely distributed (West Central) root for 'club' is *bak- (e.g. OIr bacc 'staff', Lat baculum 'staff', Grk báktron 'staff'; a Middle Dutch cognate pegge suplies NE peg); the initial b- has been explained either as the mark of a 'popular word' (i.e. one apparently used only in informal contexts and subject to the possibility of special phonological changes) or a loanword from some non-Indo-European language." ~ Mallory & Adams, The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World (2006)
bailar (Verb) "to dance"

13th cent. Borrowed from Old Occitan balar 'id.' However the appurtenance of -i- in the Spanish is inexplicable. Of note is Pyrenees dialect variant and Aragonese dialect variant bailar "to sway," from Latin baiulare "to carry a burden." Because a borrowing from Old Occitan is more likely than from a dialect oddball in Aragonese, and because the semantics in Occitan are identical to Spanish, it is easier to accept the theory that bailar is a loan from Old Occitan balar with an unexplained diphthongization. The native Old Spanish word for dancing was sotar (see jota (2)). Balar is from Latin ballare "to throw." Borrowed from Ancient Greek βάλλειν (bállein) 'id.' (see diablo).

Dialect Variants: Pyrenees balar "to sway," Asturian balar 'id.'
baile (1) m. (Noun) "dance"

Very early 14th cent. From bailar.

Also the origin of the surnames Bailes, and Bailo.
baile (2) m. (Noun) "bailiff"

Borrowed from Medieval Latin baiulivus 'id.' From Latin baiulus "porter," "steward." Of unknown origin, likely borrowed from a non-Indo-European language with the form *bagjelos "carrier."

Romance: Western Vulgar Latin: Italian baglio "beam;" Eastern Vulgar Latin: Romanian baieră "strap;" Extra-Comparanda: English bailiff (borrowed from Old French), Old French bailif "bailiff officer" (from Late Latin *baiulivus)
bajar (Verb) "to lower"

12th cent. From Vulgar Latin *bassiare 'id.' from Latin bassus "base" and Vulgar Latin verb-forming suffix -iare. See bajo.

Romance: Western Vulgar Latin: Aragonese baxar, Leonese baixare, Asturian baxar, Catalan baixar
bajo (Adjective; Adverb) "short," "low"

13th cent. From Latin bassus "base;" "fat." Borrowed from Ancient Greek βαρύς (barys) "heavy," "low." From Proto-Indo-European *gwr̥h2-u- 'id.' From *gwreh2- "to be heavy" (whence grave).

Romance: Western Vulgar Latin: Asturian baxu, Portuguese baixo, Galician baixo, Catalan baix, French bas, Italian basso

Italic: Latin gravis "heavy," Oscan bravús 'id.'

Indo-European: Germanic: Gothic kaurjos "heavy;" Indo-Iranian: Sanskrit gurú- "heavy"
bala f. (Noun) "bullet"

13th cent. Borrowed from French balle "ball," "bullet;" itself borrowed from Italian balla "ball." From a Germanic source (compare English ball). Ultimately from Proto-Germanic *ballan- 'id.' From Proto-Indo-European *bhol-on- 'id.' From a root *bhel- Kroonen (2014) hypothesizes meant "to swell up."

Italic: Latin follis "bag;" "testicles"

Indo-European: Celtic: Gaulish ballo-, Old Irish ball "penis," Middle Welsh balleg "sack;" Hellenic: Ancient Greek φαλλός (phallós) "penis"
bañar (Verb) "to bathe"

12th cent. From baño.

Romance: Western Vulgar Latin: Asturian bañar, Portuguese banhar, Galician bañar, Catalan banyar, French baigner
banco (Noun) (finance) "bank"

13th cent. From French banque 'id.,' from Italian banca "moneychanger's table," from Old Italian banca "bench." From a West Germanic language, probably Lombardic *bank (compare Old Saxon bank "bench," Old High German banc 'id.'). From Proto-Germanic *banki- 'id.' Of unknown origin. Possibly a substrate word.

Romance: Western Vulgar Latin: Asturian bancu, Portuguese banco, Galician banco, Catalan banc, French banque; Eastern Vulgar Latin: Romanian banca

Germanic: North Germanic: Old Norse bekkr; West Germanic: Old High German banc, English bench
banda (1) f. (Noun) "bandage strip," "tape"

12th cent. Borrowed from Old French bande 'id.' Borrowed from a Germanic source, perhaps Proto-West-Germanic *banda- "band," "bond." From Proto-Germanic *banda- 'id.' From Proto-Indo-European *bhondh-o- 'id.' From the root *bhendh- "to bind."

Germanic: North Germanic: Old Norse band; West Germanic: Old Saxon band, Old English beand (English band)

Indo-European: Indo-Iranian: Young Avestan baṇda- "bond"