The only free and comprehensive online etymological dictionary of the Spanish language
segundo (2) m. (Noun) (time) "second"

From Latin secundus "second (in time)" but originally the numeral "second." The sense development from a number to an increment of time derives as a metaphor from the Latin idea of an hour as composed of two parts: the second and the minute. The pars minuta prima "the first small part" was the minute and the pars minuta secunda "the second small part" was the second. For a continued etymology of the word secundus, see segundo (1).
seguramente (Adjective) "surely," "securely"

From seguro and -mente, an adjective-forming suffix.
seguridad f. (Noun) "security"

13th cent. From Latin securitatem, accusative of seguritas 'id.,' from securus "careless," "serene" (see seguro).
seguro m. (Adjective, Noun) "secure;" "insurance"

Early 13th cent. From Latin securus "careless," "serene" composed of se- "without" and cura "care." See respective entries for se- and cura for their further etymologies.

Romance: Western Vulgar Latin: Asturian seguru, Portuguese seguro, Galician seguro, Catalan segur, French sûr, Italian sicuro ; Eastern Vulgar Latin: Aromanian siguro, Romanian sigur ; Sardinian: securu
seis (Cardinal Number) "six"

12th cent. Old Spanish seis, uncommon seyes (probably under analogy from leyes). From Latin sex 'id.' From Proto-Italic *seks 'id.' From Proto-Indo-European *u̯ek-s 'id.'

Romance: Western Vulgar Latin: Asturian seis, Portuguese seis, Galician seis, Catalan sis, French six, Italian sei ; Eastern Vulgar Latin: Aromanian shasi, Romanian șase ; Sardinian: ses

Indo-European: Celtic: Celtiberian sues "six?," Old Irish 'id.,' Middle Welsh chwech 'id.,' Old Breton hue 'id.,' Cornish whegh 'id.;' Germanic: Gothic saihs "six," Old Norse sex 'id.,' Old High German sehs 'id.,' Old Saxon sehs 'id.,' English six; Balto-Slavic: Old Church Slavonic šestь "six," Russian šest' 'id.,' Czech šest 'id.,' Polish sześć 'id.,' Slovene šę̑st 'id.,' Lithuanian šešì 'id.,' Latvian seši 'id.;' Albanian: gjashtë "six;" Hellenic: Ancient Greek ἕξ (héks) "six," Doric ϝέξ (wéks) 'id.;' Armenian: vec' "six;" Indo-Iranian: Sanskrit ṣáṣ- "six," Young Avestan xšuuaš 'id.'
semana (1) f. (Noun) "week"

12th cent. Old Spanish setmana. From Latin septimana "week" (but more literally "of the seventh"), from septimus "seventh" (see séptimo) and the adjective-forming suffix -anus (see -ano).

Romance: Western Vulgar Latin: Asturian sermana, Portuguese semana, Galician semana, Catalan setmana, French semaine, Italian settimana ; Eastern Vulgar Latin: Aromanian siptãmãnã, Romanian săptămână
Semana (2) (Surname)

According to Tibón (1988), the last name Semana is from Arabic sammana "butter vender" (compare samn "butter).

Semitic: East Semitic: Akkadian šamnu(m) "oil," "fat;" Northwest Semitic: Ugaritic šmn "oil," "fat," Ancient Hebrew šemen "oil," Punic šmn 'id.'
señal f. (Noun) "sign"

10th cent. From Late Latin signale 'id.,' ultimately from signum "mark." From Proto-Italic *sekno- 'id.' From Proto-Indo-European *sek-no- "cut."

Romance: Western Vulgar Latin: Portuguese sinal, Catalan senyal, French signal, Italian segnale ; Eastern Vulgar Latin: Romanian semnal

Italic: Oscan σεγονω (segono) "statues"
señor (Noun) "lord;" (m.) "mister," (f.) "mistress"

11th cent. From Latin senior "elder," but originally "older." From senex "old." From Proto-Italic *sen-ek- 'id.' From Proto-Indo-European *sen- 'id.'

Romance: Western Vulgar Latin: Portuguese senhor, Catalan senyor, French seigneur, Italian signore ; Sardinian: sannori

Indo-European: Celtic: Gaulish Seno-gnatos "old birth," Old Irish sen "old," Middle Welsh hen 'id.,' Old Breton hen 'id.,' Germanic: Gothic sineigs "elderly;" Balto-Slavic: Lithuanian sẽnas "old," Latvian sęns 'id.;' Hellenic: Ancient Greek ἕνος (hénos) "old;" Armenian: hin "old;" Indo-Iranian: Sanskrit sána- "old," Avestan hana- 'id.'
señorito (Noun) "young person"

17th cent. From señor "lord" and diminutive -ito.