The only free and comprehensive online etymological dictionary of the Spanish language
quien, quién (Relative Pronoun) "who"
10th cent. From Latin quem "whom," accusative of qui "who." From Proto-Italic *kwim "whom." From Proto-Indo-European *kwi̯m 'id.'
South Picene pim "who," Paelignian pam 'id.'
Old Irish cía "who," Old Welsh pui 'id.,' Old Breton pou 'id.,' Cornish pyw 'id.'
Gothic ƕas "who," Old Norse hverr 'id.,' Old High German hwer 'id.,' Old Saxon hwē 'id.,' Old English hwā 'id.' (English who)
Ancient Greek τίς (tís) "who," Thessalian κις (kis) 'id.,' Mycenaean jo-qi- 'id.'
Phrygian κιν (kin) "whom"
Sanskrit kím "what," "what for," Young Avestan cim "because"
A kus "who," B kuse 'id.'
"[It is entirely possible that] *kwi-/kw- was a relative pronoun in Proto-Indo-European. Both items became relative pronouns during the histories of individual languages, possibly at a stage early enough to be considered 'dialectical Proto-Indo-European'. Most scholars taking this line conclude that Proto-Indo-European therefore had no relative clauses." ~ P. Probert, Early Greek Relative Clauses (2015)
quieto m. (Noun) "quiet"
15th cent. A learned borrowing from Latin quietus 'id.' The native word was quedo. From quiescere "to rest." From quies "rest" (see quitar) and -escere, an inchoative verb-forming suffix (see -ecer).
Asturian quietu, Portuguese quieto, Galician quieto, French quiet, Italian quieto, Romanian cet, Sardinian achietu
quitar (Verb) "to remove"
12th cent. From Vulgar Latin *quietare "to release," "to calm." From Latin quies "rest." From Proto-Italic *kwiē-ti- 'id.' From Proto-Indo-European *kweh1-ti̯- 'id.' From the root *kwi̯eh1- "to rest."
Old Church Slavonic počiti "to rest," Russian počít' 'id.,' Polish pokój "peace," Slovene počíti "to rest"
Armenian hang "rest," hang-e-aw "to be rested"
Avestan š́iiā̆- "to be calm," šāiti- "happiness"
quizá, quizás (Adverb) "maybe," "perhaps"
12th cent. Old Spanish quiçab. Probably from the common Latin phrase qui sapit "who knows." The problem is that following the ordinary laws of Spanish sound changes, qui sapit does not give us quiçab but **quisá(b). To remedy this difficulty, González Ollé (1981) hypothesizes that a phrase qui id sapit "who knows it" existed in the Iberian peninsula in Roman times which became the source. For the etymology of qui, see quien; for the etymology of sapit (3rd pers. sing. of sapere "to know") see saber.
Portuguese quiçá, Italian chissà
Originally just quizá. Quizás was created in the 16th cent. by analogy with other adverbs ending in -s.