The only free and comprehensive online etymological dictionary of the Spanish language
intentar (Verb) "to try"
From Latin intentar "to extend toward," from intendere "to focus on," "to strain" (see entender) and frequentive suffix -tare (see note under faltar).
Indo-European
Romance
Asturian entender, Portuguese intender, Catalan entendre, French entendre, Italian intendere, Aromanian ntindu, Romanian întinde, Sardinian intèndhere
intento m. (Noun) "attempt"
Meaning influenced by Medieval Latin intentus "attempt." From Latin intentus "extension;" itself from intentare "to extend towards" (see intentar).
inter- (Prefix) "between"
From Latin inter- 'id.' Akin to English inter-, which was borrowed from the Latin. Originally a preposition inter (see entre).
interesante (Adjective) "interesting"
17th cent. From interesar.
interesar (Verb) "to interest"
17th cent. From Medieval Latin interesse "involvement," from Latin interesse "involvement" but originally "to be between." From inter- and esse "to be" (see entre and ser).
ínterin m. (Noun, Adverb) "interim," "meanwhile"
From Latin interim 'id.' From inter "between" (see entre) and -im, "toward that" (accusative of is, see éste).
interior m. (Adjective, Noun) "interior"
15th cent. An adjectival from Latin interior 'id.' From Proto-Italic *n̥tero- 'id.' From Proto-Indo-European *h1én-tero- 'id.,' a compound of *h1en "in" (whence en) and *-tero-, a comparative suffix (see -tr-).
Indo-European
Celtic
Celtiberian entara, Old Irish eter, Old Welsh ithr, Breton etre, Cornish ynter
Germanic
Gothic undar "between," Old Norse undorn "midday," Old Saxon under "before midday," Old High German untar "between," Old English under "before midday" (English in)
Albanian
Albanian ndër
Balto-Slavic
Lithuanian į́sčios "womb," Latvian ìekšas "entrails," Old Prussian instran "fat"
Hellenic
Ancient Greek ἔντερα (éntera) "intestines"
Armenian
Armenian ənderk' "intestines"
Indo-Iranian
Sanskrit antár, Old Avestan aṇtarə̄̆
-into (Suffix) Fossilized suffix from Ancient Greek -ινθος (-inthos), found in a number of loanwords from the Pre-Greek language (contrast a loanword with the suffix: Ancient Greek λαβύρινθος (labyrinthos) "labyrinth;" and without: Lydian labrys "axe").

From Pre-Greek *-indh-, some sort of suffix fixed to nouns to make new nouns. Recently, Kroonen (2012) has convincingly demonstrated cognate fossils in Germanic, Albanian, Armenian, and Celtic.
Indo-European
Celtic
Middle Irish -ed, from a source language as *-ind-
Germanic
All from a non-Indo-European language *-ĩd-, which comes from Pre-Greek *-indh-, either because Pre-Greek had evolved since the time that Ancient Greek was in contact or due to an intermediary language distorting the sound. Old Norse -t (cf. ertr "peas" (found only in plural)), -it (cf. gnit "nit"), Old High German -īz, -eiz, Old Saxon -it, Middle Low German -ete, Middle Dutch -ete
Albanian
Albanian -í, from *-id-
Armenian
Armenian -ic, from a source language as *-id
intoxicación f. (Noun) "intoxication"
From intoxicar.
intoxicar (Verb) "to intoxicate"
Very early 14th cent. Old Spanish entoxicar. From Latin in- (2) and toxicare (see toxicar).