The consonant cluster/ks/ was, in Ancient Greek, written as either ChiΧ (Western Greek) or XiΞ (Eastern Greek). In the end, Chi was standardized as /kʰ/ (/x/ in Modern Greek), while Xi was standardized for /ks/. But the Etruscans had taken over Χ from older Western Greek; therefore, it stood for /ks/ in Etruscan and Latin.
It is unknown whether the letters Chi and Xi are Greek inventions, or whether they are ultimately of Semitic origin. Chi was placed toward the end of the Greek alphabet, after the Semitic letters, along with Phi, Psi, and Omega, suggesting that it was an innovation; further, there is no letter corresponding specifically to the sound /ks/ in Semitic. There was a Phoenician letter kheth with a probable sound /ħ/, somewhat similar to /kʰ/, but this was adopted into Greek as first the consonant /h/, and later, the long vowel Eta (Η,η), and does not seem to have been the source of Greek Chi. The Phoenician letter Samekh (representing /s/) is usually considered the inspiration for Greek Xi, but as noted, Chi had a graphically distinct shape from Xi — although it may possibly have been another variant originally based on samekh. The original form of samekh may have been an Egyptian hieroglyph for the Djed column, but this too is uncertain, as no intervening Proto-Sinaitic form of this letter is attested.
Egyptian hieroglyph "column"
Phoenician S
Greek Xi
Greek Chi
Etruscan X
Usage
In some languages, as a result of assorted phonetic changes and handwriting adaptations, X has other pronunciations:
Dutch: The island of Texel is pronounced as Tessel. This is because ss was written with a ligature closely resembling the x.
English: X is a double consonant or, rather, a sign for the compound consonants [ks]; or sometimes when followed by an accented syllable beginning with a vowel, or when followed by silent h and an accented vowel [gz] (e.g. exhaust, exam); usually [z] at the beginnings of words (e.g. xylophone), and in some compounds keeps the [z] sound, as in (e.g. meta-xylene). It also makes the sound [kʃ] in words ending in -xion (typically used only in British-based spellings of the language; American spellings tend to use -ction). It can also represent the sounds [gʒ] or [kʃ], for example, in the words luxury and sexual, respectively. When the letter X begins a word in the English language such as xynene and a z sound is created the X is said to be silent. Final x is always [ks] (e.g. ax/axe) except in loan words such as faux (see French, below).
French: at the ends of words, silent (or [z] in liaison if the next word starts with a vowel). This usage arose as a handwriting alteration of final -us. Two exceptions are pronounced [s]: six and dix.
In Italian, X is always pronounced [ks], as in the words ex, extra, xilofono.
In Norwegian, X is generally pronounced [ks], but since the nineteenth century there has been a tendency to spell it out as ks whenever possible; it may still be retained in names of people, though it is fairly rare, and occurs mostly in foreign words and SMS language.
Spanish: In Old Spanish, X was pronounced like [ʃ] as it is still currently in other Iberian languages. Later, the sound evolved to a hard [x] sound. In modern Spanish, the hard [x] sound is spelled with a j, or with a g before e and i, though x is still retained for some names (notably México, which alternates with Méjico). Now, X represents the sound [s] (word-initially), or the consonat clusters [ks] and [gs] (e.g. oxígeno, examen). Even rarer; like in Old Spanish, the x can be pronounced as [ʃ] in modern day in some proper nouns such as Raxel (a variant of Rachel) and Xelajú. In American and seseo Spanish, the xc in excelente is pronounced as [ks] but in Spain, this combination is pronounced [ks-θ].
In Albanian, x represents [dz], while the digraphxh represents [dʒ].
Polish doesn't use X. In loanwords, X is either replaced by ks like in 'ekstra' (extra), or gz like in 'egzotyczny' (exotic).
No words in the Basic English vocabulary begin with X, but it occurs in words beginning with other letters. It is often found in a word with an E before it. Otherwise it is the third most rarely used letter in the English language.
In musical notation, a double sharp, raising a note by one full step
In electronic circuit diagrams, an unused pin of a chip, or, in a circle, a lamp
Short for "cross", including such abbreviations as "lax" for "lacrosse", or "x-country" for "cross country"
Generally speaking, Americans do not refer to a lone "x" as a cross, where as this usage does occur outside of the U.S., such as in the name noughts and crosses, which Americans call tic-tac-toe.
In Microsoft Windows, X is on a button that closes a window. X as a verb means means to close a window.
In algebra, X is very commonly associated with an unknown variable. Its first time use is attributed to Omar Khayyám in his Treatise on Demonstration of Problems of Algebra (1070).[2]
The X axis means the horizontal axis in graphs.
In France "L'X" is the nickname of the prestigious engineer school Polytechnique
In the MPAA rating system, X used to indicate an adults-only film. The rating, which was not trademarked, is now widely used to denote pornographic films.
References
^ "X" Oxford English Dictionary, 2nd edition (1989); Merriam-Webster's Third New International Dictionary of the English Language, Unabridged (1993); "ex," op. cit.
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kheth with a probable sound 
Samekh (representing /s/) is usually considered the inspiration for Greek Xi, but as noted, Chi had a graphically distinct shape from Xi — although it may possibly have been another variant originally based on samekh. The original form of samekh may have been an 
