The only free and comprehensive online etymological dictionary of the Spanish language
investigación f. (Noun) "investigation"

From Latin investigationem, accusative of investigatio 'id.' From the verb investigare "to investigate" (see investigar).

Romance: Western Vulgar Latin: Italian investigazione
investigar (Verb) "to investigate"

From Latin investigare 'id.' From in- "in" (see in- (2)) and vestigare "to follow a track;" from vestigium "track" (see vestigio).

Romance: Western Vulgar Latin: French investiguer
-ío Noun-forming suffix to form abstract nouns from verbs.

From Latin -io, originally an -o suffix on i-stem verbs.
ir (Verb) "to go"

11th cent. The modern Spanish verb, infamously irregular, is a collapse of multiple Latin verbs into a single word. Forms beginning with i- (ir, iba, etc…) derive from Latin ire 'id.' Forms with v- (voy, vaya, etc…) are from vadere "to go," "to walk." Forms beginning with f- (fui, fueras, etc...) were borrowed from ser respectively (see fui and ser for continuing etymology of these forms). Latin ire is from Proto-Italic *ei- "to go." From Proto-Indo-European *h1ei̯- 'id.' Latin vadere "to go," "to proceed" is from Proto-Italic *wāþ-e/o- 'id.' A thematic present from Proto-Indo-European *u̯eh2dh- 'id.'
-ísimo Superlative suffix.

From Latin -issimus 'id.' From Proto-Italic *-ism̥mo- 'id.' with a post-Proto-Italic expressive lengthening of the -s- to -ss-. A compound of *-is-, a comparative suffix, and *-(t)m̥mo-. For the origin of *-(t)m̥mo-, see -tr-. Proto-Italic *-i̯s- is from Proto-Indo-European *-i̯s-to-, the original superlative suffix in Proto-Indo-European.

Romance: Western Vulgar Latin: Portuguese -íssimo, Catalan -íssim, French -issime, Italian -issimo

Italic: Paelignian -ismu (-isamo), Oscan -imas (pre-form *-ismo-), Umbrian -ime (pre-form *-ismo-)

Indo-European: Celtic *-isamo-: Old Irish -(i)ssam, Middle Welsh -af, Breton -aff, Celtiberian (f.) -sama, Gaulish (f.) -xama

"Now, 'expressive' [lengthening] has been invoked frivolously over the years, and is little more than a gong announcing the lack of a real explanation. But in this case for once it is not far-fetched. The superlative adjective has a distinct, and salient, semantic 'push', an emphasis which is often moreover conscious, like the emphatic aspirated stops in French in place of the otherwise plain articulation of such consonants. Besides, one might suppose that the prosodics of the specific form pessimus 'worst'... would inevitably contribute to the ultimate success of the ending -(i)ssimus." ~ A. Sihler, New Comparative Grammar of Greek and Latin (2008)
isla f. (Noun) "island"

13th cent. From Latin insula 'id.' Of unknown origin. Presumably borrowed from a non-Indo-European language.

As for the origin of the surnames de la Isla, Islas, Islán, and Isoler, they originally indicated that the individual hailed from the town of Isla, Cantabria.

Romance: Western Vulgar Latin: Asturian isla, Portuguese isla, Galician insua, Catalan illa, French île, Italian isola; Eastern Vulgar Latin: Romanian insulă ; Sardinian: isula
-ito Diminutive suffix.

From Vulgar Latin *-ittus 'id.' Of unknown origin. Variation in Vulgar Latin *-itus ~ *-ittus may parallel Aquitaine Basque *-to- ~ *-tto-. In which case we are dealing with a loan from a non-Indo-European language.

Romance: Western Vulgar Latin: Asturian -etu, Portuguese -ito, Catalan -et, Occitan -et French -et, Italian -itto; Extra-Comparanda: English -et (borrowed from French)

"Certainty, of course, is not attainable in the study of such obscure elements of language [such as *ittus]... these phenomena, which, though not quite unnoticed, are far from having hitherto received due attention on the part of Romance scholars." ~ A. J. Carnoy, "Apophony and Rhyme Words in Vulgar Latin Onomatopoeias" (1917)
-ivo Adjective-forming suffix.

From Latin -ivus 'id. From Proto-Italic *-īwos 'id.' From Proto-Indo-European *-i̯H-u̯ós 'id.'

Romance: Western Vulgar Latin: Asturian -ivu, Portuguese -ivo, Catalan -iu, French -if, Italian -io; Eastern Vulgar Latin: Romanian -iu; Extra-Comparanda: English -ive (borrowed from Anglo-Norman -if)
izquierda (Adjective) "left"

12th cent. Old Spanish exquerdo. From Basque ezkerdo "left-handed" and thus from ezker(r) "left" but this etymology is only superficially convenient as a suffix -do is of unclear origin. It may not come from ezker(r) at all but esku "half" or Celtic *kerros "left" (though the latter has fell out of scholastic favor in recent years). The details and arguments exceed the scope of this dictionary. For a fuller discussion, see Zytsar (2000).