Xjyuk

The official name of the letter X in the old Russian alphabet was хѣръ, which is how modern Russians call a dick.

The names of most other letters of the old Russian alphabet are understandable: азъ (I), буки (letter), вѣди (know), глаголь (speak), добро (good), etc.

What is the origin of the letter name хѣръ?

UPDATE: To ensure that everyone understand my question right, I want to emphasize that my question is about why the letter itself was called хѣръ, not about why it is nowadays an obscene word. I know that another obscene word starts with X and that people started using the letter name to refer to that word, so the letter name itself became an obscene word, too. The name for the letter was obviously chosen well before this name became an obscene word.

UPDATE 2: I am aware of the херувим hypothesis and strongly dislike it, for the old names of Russian letters are generally complete and are not abbreviations: живѣ́те, глаголь, мыслѣ́те. It is hard to imagine that the authors of the letter names abbreviated херувим, because this word has the same number of syllables as живѣ́те and мыслѣ́те, which were not abbreviated. Moreover, the very idea of the old names of Russian letters was to provide a complete word that starts with a given letter. Abbreviating a word to something unrecognizable is just contrary to the spirit of the old names of Russian letters. I thus strongly suspect that хѣръ had its own meaning understanable to at least a part of the Russians of that epoch. Some mysterious forgotten meaning. And my primary motivation to ask the question was to find that meaning.

The matter is that the letter Х is the first letter of the Russian obscene word, so the word was shortened in speech to just this letter called by its alphabetic name. So, хѣръ in this meaning was initially an abbreviation, then it became an euphemism, and only afterwards it came to being a synonym.

Actually nobody knows for sure. What we definitely know - it has nothing to do with obscene хер. Well, only in that sense that obscene "хер" might be a derivative from the letter as an euphemism of "хуй". It also sound similar to other derivative - хрен.

Even more - there's a totally non-obscene verb похерить which means to obliterate, to cancel out, figuratively - to lose something.

Here's a quote from wikipedia:

В старо- и церковнославянской азбуках носит название «хѣръ», смысл
которого не ясен: считать его, как это часто делается, связанным со
словом «херувим» затруднительно (последнее не содержало ятя, хотя в
качестве фонетической адаптации мягких заднеязычных ять мог иногда
появляться в заимствованиях, например, известны написания вроде
Гѣръманъ)

One alternative definition we can find in Max Fasmer's famous dictionary:

Кроме того, допускают происхождение из греч. χαῖρε "привет тебе"

Keep in mind, however, that this dictionary is as much famous as in some aspects quite obsolete.

As a side note: while the majority of Cyrillic letters indeed have names that are basically valid Slavic words - even рцы - which is not that obvious for modern Russian speaker - still, there are some letters named not after any word - like цы for instance.