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

From demás and -ía.
demasiado (Adjective) "too much," "excessively"

From demasía and -ado.

Romance: Western Vulgar Latin: Asturian demasiáu, Portuguese demasiado, Galician desmasiado
demonio m. (Noun,) "devil," "demon"

13th cent. From Latin daemonium 'id.,' borrowed from Ancient Greek δαιμόνιον (daimónion) "spirit," from δαίμων ‎(daímon) "god." From late Proto-Indo-European *deh2-i̯-mo- "divided (from the people)," *deh2-i̯- "to cut."

Romance: Western Vulgar Latin: Asturian demoniu, Portuguese demónio, Galician demoño, Catalan dimoni, French demoygne, Italian demonio; Eastern Vulgar Latin: Romanian demon

Hellenic: Ancient Greek δαίομαι (daíomai) "divider"

Indo-European: Balto-Slavic: Old Church Slavonic bogъ "god," Russian bog 'id.,' Polish bóg 'id.,' Slovene bọ̑g 'id.,' Old Prussian baga- 'id.;' Indo-Iranian: Sanskrit bhájati "to divide," bhága- "prosperity," Young Avestan baɣa- "lord"
dentro (Preposition) "within"

11th cent. From Latin de intro "from inside" (see de and entrar respectively).

Romance: Western Vulgar Latin: Asturian dientro, Portuguese dentro, Galician dentro, Italian dentro
departamento m. (Noun) "department"

Borrowed from French département 'id.' From départir "to separate," from de- "from" (see de-) and partir "to part" (see partir).
depender (Verb) "to depend"

From Latin dependere "to hang from." From de- "from" and pendere "to hang." See de- and pender respectively.
derecha (Adjective) "right (side)"

10th cent. From Vulgar Latin *derecta, from Latin directa "straight" (see derecho).

Romance: Western Vulgar Latin: Asturian derechu, Portuguese direito, direto, Catalan dret, French droit, direct, Italian diritto, diretto; Eastern Vulgar Latin: Aromanian ãndreptu, ndreptu, Romanian drept, direct; Sardinian: daretu, deretu

Having all but replaced the native Latin word dexter, which survives in diestro.
derecho m. (Adjective, Noun) "straight;" "(legal) right," "law"

11th cent. From Vulgar Latin *derecto, from Latin directus "straight." As a noun referring to human or political rights, the idea comes from the French Revolution, when the political conservatives sat on the right parliamentary wing. Latin directus comes from dis- (see des-) and regere "to rule" (see regir).

Romance: Western Vulgar Latin: Asturian derechu, Portuguese direito, Galician dereito, Catalan dret, French droit, Italian diretto; Eastern Vulgar Latin: Aromanian dreptu, Romanian drept; Sardinian daretu
derribar (Verb) "to demolish;" "to flagellate"

From Vulgar Latin *derripare "from the river bank." A compound of Latin de "from" (see de) and ripa "river bank" riba
desaparecer (Verb) "to disappear"

From des- and aparecer.