Lewis & Short

X, x, a character probably derived from the Greek Ξ (this form of that letter being found in some few Greek inscriptions). Though not introduced instead of the characters for the two separate sounds till after the adoption of the alphabet, the letter x is certainly older than the Latin inscriptions known to us; for we find in the Columna rostr., EXEMET MAXIMOS, EXFOCIONT; in the fifth Epitaph of the Scipios, SAXSVM; and in the S. C. de Bacch., EXDEICENDVM, EXDEICATIS. EXTRAD, etc.The sound of X was like that of the Greek ξ, i. e. ks, although etymologically it represented not only cs (as in lux, from luc-s, and dixi, from dic-si), but also gs (as in lex, from leg-s; rexi, from reg-si); hs (as in traxi, from trah-si; vexi, from vehsi); and chs (as in the word onyx, from onych-s, borrowed from the Greek). The hardening of a softer final (g, h, ch) before s into the c-sound, which occurs in the last-mentioned cases, is found also in several roots ending in v and u: nix for niv-s, vixi for viv-si, connixi for conniv-si, fluxi for fluv-si, from fluo (root FLUV; cf. fluvius), struxi for stru-si. Less frequently x has arisen from the combinations ps and ts: proximus for prop-simus (from prope), nixus for nit-sus (from nitor), the latter being used along with the collateral form nisus, as also connivi with connixi, and mistus (from misceo) with mixtus. An exchange of the sounds ss, or s and x, took place in axis for assis, laxus for lassus; cf. also Ulixes, from the Sicilian Οὐλίξης, Etruscan Uluxe for Ὀδυσσεύς; so, too. Sextius, Exquiliae = Sestius, Esquiliae; cf. also Ajax = Αἴας. In the later language of the vulgar, the guttural sound in x disappeared, and s or ss was often written for it; as vis for vix. vixit for visit. unsit for unxit, conflississet for conflixisset, in late Inscrr. (v. Corss. Ausspr. I. p. 297 sq.); hence regularly in Italian, and frequently in the other Romance tongues, the Lat. x is represented by s or ss. Respecting the nature of x in composition, v. ex.By a mere graphic variation, one of the constituent sounds of x is often expressed in inscriptions (but not the earliest, v. Corss. Ausspr. I. p. 296) by an additional c or s; as SACXO or SAXSO for saxo; VCXOR or VXSOR for uxor; CONIVNCX or CONIVNXS for conjux; even both sounds are sometimes thus expressed, VICXSIT for vixit.As an abbreviation X stands for decem, ten; it was stamped upon the silver denarius, so called because it was valued at ten asses.