Let the creators of TeX and LaTeX answer:

Donald Knuth wrote in the first chapter of his TeXbook:

English words like ‘technology’ stem from a Greek root beginning with the letters τεχ...; and this same Greek word means art as well as technology. Hence the name TeX, which is an uppercase form of τεχ.

Insiders pronounce the χ of TeX as a Greek chi, not as an ‘x’, so that TeX rhymes with the word blecchhh. It’s the ‘ch’ sound in Scottish words like loch or German words like ach; it’s a Spanish ‘j’ and a Russian ‘kh’. When you say it correctly to your computer, the terminal may become slightly moist.