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.