The only free and comprehensive online etymological dictionary of the Spanish language
joven (Noun) "youth"

13th cent. From Latin iuvenis 'id.' From Proto-Italic *juwen- 'id.' From Proto-Indo-European *h2i̯u̯-h1en- "young," but more literally "one full of life's energy." From *h1oi̯-u̯- "lifetime" (see evo).

Also the origin of the surname Joven, sometimes indicating the person was under the age of 25.

Romance: Western Vulgar Latin: Asturian xoven, Portuguese jovem, Galician xove, Catalan jove, French jeune, Italian giovane; Eastern Vulgar Latin: Aromanian gioni, Romanian june; Sardinian: giovanu

Indo-European: Celtic: Old Irish óetiu "youth;" Germanic: Gothic jund- "youth," Old High German jugund 'id.,' Old Saxon juguth, Old English geogoð (English youth); Balto-Slavic: Old Church Slavonic junъ "young," Lithuanian jáunas 'id.;' Indo-Iranian: Sanskrit yúvan- "youth," Young Avestan yuuānəm 'id.'
juego m. (Noun) "game"

12th cent. From Latin iocus "joke," "game." From Proto-Italic *joko- 'id.' From Proto-Indo-European *i̯ok-o- "word."

Romance: Western Vulgar Latin: Asturian xuegu, Portuguese jogo, Galician xogo, Catalan joc, French jeu, Italian gioco; Eastern Vulgar Latin: Aromanian gioc, Romanian joc; Sardinian: giogu

Italic: Umbrian iuca "prayers," Paelignian iocatin "to command"

Indo-European: Celtic: Middle Irish icht "people," Middle Welsh ieith "language," Breton yezh 'id.;' Germanic: Old High German jiht "confession"
juez (Noun) "judge"

12th cent. Old Spanish júdez, júez. From Latin iudex "judge," but more literally "law-saying." From Proto-Italic *jows-diks 'id.' From *jowo- "law" (see jurar) and *diḱ- "saying" (see -ez).

Competing feminine forms la juez / la jueza is from uncertainty due to the recent inclusion of women into the predominantly male profession. More recently, la juez has come to mean "female judge" while la jueza retains the older meaning of "judge's wife." The sense of a wife's judge is from a pre-21st cent. idea that only men could serve as judges. Also the origin of the surname Juez.

Romance: Western Vulgar Latin: Asturian xuez, Portuguese juiz, Galician xuíz, Catalan jutge, French juge, Italian giudice; Eastern Vulgar Latin: Romanian jude; Sardinian: giuge

"The Latin compound iu-dex implies the notion of showing with authority... The whole history of Lat. dicere highlights a mechanism of authority: only the judge can dicere ius." ~ E. Benveniste, Indo-European Language and Society (1973)
jugar (Verb) "to play (a game)"

12th cent. From Vulgar Latin iocare 'id.,' from Latin iocari "to joke." Derived from the noun iocus "joke" (see juego).

Romance: Western Vulgar Latin: Asturian xugar, Portuguese jogar, Galician xogar, Catalan jugar, French jouer, Italian giocare; Eastern Vulgar Latin: Aromanian gioc, Romanian juca; Sardinian: giocae
juicio m. (Noun) "judgement"

12th cent. From Latin iudicium 'id.,' from iudex "judge" (see juez).

Romance: Western Vulgar Latin: Asturian xuiciu, Portuguese juízo, Catalan judici, Italian giudicio; Eastern Vulgar Latin: Aromanian giudets, Romanian județ; Sardinian: giudísciu
junto (Adjective) "joined"

15th cent. From Latin iunctus 'id.,' from iungere "to join." From Proto-Italic *jung-e/o- 'id.' From Proto-Indo-European *i̯u̯-n-e-g- 'id.' A thematic verb (originally *i̯u̯-n-g-) derived from the root *i̯eu̯g- "to join," "to yolk" (see yugo) and the mysterious nasal-infix *-n- of unknown origin that formed the present tense in athematic verbs.

Romance: Western Vulgar Latin: Asturian xuntu, Portuguese junto, Galician xunto, Catalan junt, French joint, Italian giunto
jurar (Verb) "to swear (an oath)"

12th cent. From Latin iurare 'id.' From Proto-Italic *jowes-ā-je 'id.' From the noun *jowes- "oath." From Proto-Indo-European *h2oi̯-u̯- "lifetime," "life force" (see joven).

Romance: Western Vulgar Latin: Asturian xurar, Portuguese jurar, Galician xurar, Catalan jurar, French jurer, Italian giurare; Eastern Vulgar Latin: Aromanian giur, Romanian jura; Sardinian: giurài

Indo-European: Celtic: Old Irish uisse "fitting"
justicia f. (Noun) "justice"

12th cent. From Latin iustitia 'id.,' from iustus "just" (see justo).

Romance: Western Vulgar Latin: Asturian xustica, Portuguese justiça, Galician xustiza, Catalan justícia, French justice, Italian giustizi; Eastern Vulgar Latin: Romanian justiție
justo (Adjective) "just," "fair"

12th cent. From Latin iustus 'id.' From Proto-Italic *jowes-to- 'id.' Potentially a somewhat late adjective derived from *jowes- "oath." For a continued etymology, see jurar.

The word is the origin of a number of surnames, Justo, Justa, Justal, Juste, Justos, Justes. Also the origin of Justel, a small village in Leon.

Romance: Western Vulgar Latin: Asturian xustu, Portuguese justo, Galician xusto, Catalan just, French juste, Italian giusto; Eastern Vulgar Latin: Aromanian giustu, Romanian just; Sardinian: giustu