The only free and comprehensive online etymological dictionary of the Spanish language
se (1) (Pronoun) "oneself"

From Latin se 'id.' From Proto-Italic *se 'id.' From Proto-Indo-European *se 'id.'

In Proto-Indo-European, *s(u̯)e- meant "peculiar to oneself." Derivations from *su̯e- thus connote distinctiveness from other things. Benveniste (1973) writes, "This duality survived, as is revealed by the etymology, in the two forms se of Latin, which have become independent; the reflexive se, indicating “self,” and the separative se-, sed ‘but’, marking distinction and opposition."
se- (1), sed- Partitive prefix. "itself"

From Latin se(d)- 'id.' From se "oneself" (see se (1)).
se (2) (Pronoun) "him," "her," "them"

Old Spanish gelo. From Vulgar Latin gelo 'id.,' from Latin illi "to him." According to Roberts (2014), Old Spanish g- to Modern Spanish s- under influence from se (1).
se- (2) Prefix. "away"

From Latin se- 'id.' From Proto-Italic *sed "by itself." The ablative of se "itself" (see se (1), se (2)).
sección f. (Noun) "section"

From Latin sectionem, accusative of sectio 'id.' From secare "to cut." From Proto-Italic *seka-je/o- 'id.' From Proto-Indo-European *sekh1-i̯e/o- "to cut." Probably borrowed from another language.

Romance: Western Vulgar Latin: Portuguese secção / seção, French section, Italian sezione

Semitic: Ancient Hebrew śakkīn "slaughtering knife," Aramaic sakkīn 'id.' (both likely borrowed)

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"
secreto (Adjective) "secreto"

13th cent. From Latin secretus 'id.,' passive participle of secernere "to separate," from se- "itself" and cernere "to discern" (see cerner).

Romance: Western Vulgar Latin: Asturian secretu, Portuguese secreto, Galician secreto, Catalan secret, French secret, Italian secreto ; Eastern Vulgar Latin: Romanian secret ; Sardinian: segretu
seguir (Verb) "to continue," "to follow"

11th cent. From Vulgar Latin sequere, from Latin sequi 'id.' From Proto-Italic *sekw-e/o- 'id.' From Proto-Indo-European *sekw-e/o- 'id.'

Romance: Western Vulgar Latin: Portuguese seguir, Catalan seguir, French suivre, suite, Italian seguire

Indo-European: Celtic: Old Irish sechithir "to follow;" Germanic: Gothic saiƕan "to see," Old Norse sjá 'id.,' Old High German sehan "to see," Old Saxon sehan 'id.,' English to see; Balto-Slavic: Lithuanian sèkti "to follow," Latvian sekt 'id.;' Hellenic: Ancient Greek ἕπομαι (hépomai) "to follow;" Indo-Iranian: Sanskrit sácate "to follow," Avestan haca- "to accompany"
según (Preposition) "according to"

13th cent. Apocopation of an unattested pre-form *segundo. From Latin secundum 'id.' From secundum "after," from secundus "second," "following" (see segundo (1)).
segunda f. (Noun) "double turn of a lock;" (singular & plural) "double meaning"

From segundo (1).
segundo (1) m. (Adjective, Noun) "second"

13th cent. Old Spanish segundo, rarely secundo; apocopated form segund, segunt. From Latin secundus 'id.' From Proto-Italic *sekwo-ndo- "second," but literally "the following." See seguir.

Romance: Western Vulgar Latin: Portuguese segundo, Catalan segon, French second, Italian secondo ; Eastern Vulgar Latin: Romanian secund