The only free and comprehensive online etymological dictionary of the Spanish language
sentar (Verb) "to seat"

12th cent. From Vulgar Latin sedentare 'id.,' from sedens "seating," from sedere "to sit." Prior to the 12th cent., far more common than sentar was assentar, from Vulgar Latin *adsentare. From Proto-Italic *sizd-e/o- 'id.' From Proto-Indo-European *si̯-sd-e/o- "to be sitting." Reduplication of *sed- "to sit."

Romance: Western Vulgar Latin: Asturian sentar, Portuguese sentar, Galician sentar, Catalan seure, French seoir, Italian sedere ; Eastern Vulgar Latin: Aromanian shed, Romanian ședea ; Sardinian: sèdere

Italic: Umbrian sistu "he sat down"

Indo-European: Balto-Slavic: Old Church Slavonic sěsti "to sit down," Russian sest' 'id.,' Czech siesti 'id.,' Polish siąść 'id.,' Slovene sẹ́sti 'id.,' Old Prussian sindants "sitting," Lithuanian sė́sti 'id.;' Hellenic: Ancient Greek ἵζειν (hízdein) "to sit down;" Indo-Iranian: Sanskrit sī́dati "to sit," Avestan hiδa- 'id.'
sentido (Adjective, Noun) "felt;" "sense"

11th cent. Old Spanish sentido. From sentir.

Romance: Western Vulgar Latin: Asturian sentir, Portuguese sentir, Galician sentir, Catalan sentir, French sentir, Italian sentire ; Eastern Vulgar Latin: Aromanian simtu, Romanian simți ; Sardinian:sentire
sentir (Verb) "to feel"

12th cent. Old Spanish sentir. From Latin sentire 'id.' From Proto-Italic *senti-je/o- 'id.' From Proto-Indo-European *sn̥t-i̯e/o- 'id.' From a root *sent- "to feel," "to think."

Romance: Western Vulgar Latin: Asturian sentir, Portuguese sentir, Galician sentir, Catalan sentir, French sentir, Italian sentire ; Eastern Vulgar Latin: Aromanian simtu, Romanian simți ; Sardinian:sentire

Indo-European: Balto-Slavic: Church Slavic sęštъ "wise," Lithuanian sintė́ti "to think"
séptimo (Ordinal Number) "seventh"

17th cent. loanword from Latin septimus 'id.,' from septem "seven" (see siete). It replaced the native word siedmo.

Romance: Western Vulgar Latin: Portuguese sétimo, French septime, Italian settimo
ser (Verb) "to be"

10th cent. Old Spanish seer. A collapse of Vulgar Latin sedere "to sit" and essere "to be" into a single verb. Already by the 4th cent., Iberian Vulgar Latin sedere was employed in the manner of Modern Spanish ser and not in the sense of sitting. For the etymology of sedere, see notes under sentar. Essere is from Latin esse 'id.' From Proto-Italic *es- 'id.' From Proto-Indo-European *h1es- 'id.'

Romance: Western Vulgar Latin: Asturian ser, Portuguese ser, Galician ser, Catalan seure, French seoir, Italian sedere ; Eastern Vulgar Latin: Aromanian shideari, Romanian ședere ; Sardinian: sèere
serie f. (Noun) "series"

Very late 15th cent. From Latin series 'id.' from serere "to tie together." From Proto-Italic *ser-e/o- 'id.' see cerrar.

Romance: Western Vulgar Latin: Portuguese série, Catalan sèrie, French série, Italian serie

Italic: Oscan aserum "to lay"

Indo-European: Germanic: Gothic sarwa "armor," Old Norse sørvi "collar," Old High German saro "armor," Old English searu "art;" Balto-Slavic: Old Lithuanian sėris "thread;" Hellenic: Ancient Greek εἴρειν (eírein) "to knit," "to tie;" Tocharian: A ṣurm "motive," B ṣarm 'id.'
serio (Adjective) "serious"

17th cent. borrowing from Latin serius 'id.' From Proto-Italic *sēr-jo- 'id.' From Proto-Indo-European *seh1-ro- "heavy."

Romance: Western Vulgar Latin: Portuguese sério, French sérieux

Indo-European: Germanic: Gothic swers "honored," Old Norse svárr 'id.,' Old High German swār 'id.,' Old Saxon swār 'id.,' Old English swǣr 'id.;' Balto-Slavic: Lithuanian svarùs "heavy," Latvian svars "weight;"
servicio m. (Noun) "service"

10th cent. Borrowed from Latin servitium "slavery," from servus "slave" (see siervo).

Romance: Western Vulgar Latin: Portuguese serviciu, French service, Italian servigio, servizio ; Eastern Vulgar Latin: Romanian serviciu
servir (Verb) "to serve"

10th cent. From Latin servire 'id.,' from servus "servant" (see siervo).

Romance: Western Vulgar Latin: Asturian servir, Portuguese servir, Galician servir, Catalan servir, French servir, Italian servire ; Eastern Vulgar Latin: Romanian șerbi, servi ; Sardinian: selvire
sexo m. (Noun) "sex"

14th cent. borrowing from Latin sexus 'id.' Probably from Proto-Italic *seks- 'id.,' and that from Proto-Indo-European *sekh1-s- "to cut."

Romance: Western Vulgar Latin: Portuguese sexo, French sexe, Italian sesso ; Eastern Vulgar Latin: Romanian sex

Indo-European: Balto-Slavic: Old Church Slavonic sěšti "to cut," Russian seč' 'id.,' Czech síci "to mow," Polish siec "to beat," Slovene sẹ́či "to cut;" Anatolian: Hittite šākk-i "to know"