custos.

Of unclear ultimate origin. The mainstream etymology, proposed by Nowicki (1978), derives the word from a compound *kusto-sd- (“who sits near the hidden/near the treasure”), where the first element is cognate to Proto-Germanic *huzdą (“hidden treasure”) (also of uncertain etymology; see more at English hoard and below) and the second element is a reduced form of Proto-Indo-European *sed- (“to sit”). However, this etymology is disputed, and De Vaan is skeptical of it, with further discussion in the below box. More information One proposal derives *kusto- and *huzdą from *kudʰ-to-, from Proto-Indo-European *(s)kewdʰ- (“to cover, wrap, encase”), from *(s)kewH- (“to cover, hide”), whence Ancient Greek κεύθω (keúthō, “to conceal”) and Old English hȳdan (“to hide, conceal, preserve”). But the derivation of Latin /st/ and Proto-Germanic *zd from PIE *dʰ-t is controversial. Some etymologists consider /st/ to be the regular outcome in Latin of PIE *dʰ-t, based on aestās and aestus; according to this view, the /ss/ found in participle forms such as iussus and fossus was introduced by analogy with forms built on stems ending in other dental consonants. But De Vaan argues that it is better to analyze /ss/ as the regular outcome of *dʰ-t, and the /st/ of aestās and aestus as an analogical reformation. Thus, De Vaan considers the /st/ in custōs unexplained and the etymology unknown. Oswald Szemerényi (1952/3 [1987]) alternatively derives the /st/ here and the *zd in Proto-Germanic *huzdą from PIE *sdʰ, tracing both back to a Proto-Indo-European *k(e)wsdʰo- (“palace, treasure house, treasure”). This etymology is accepted by Ringe (2006) who mentions Ancient Greek κύσθος (kústhos, “vulva”) as another cognate. However, Lubotsky (2004) finds this unconvincing and considers the regular outcome of *sdʰ in Latin to be a long vowel + /d/.