The official name of the letter Х in the old Russian alphabet was хѣръ, which is how modern Russians say хер (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 correctly, I want to emphasize that it's about why the letter itself was called хѣръ, not about why it is an obscene word nowadays. I know that another obscene word starts with Х 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 given that the old names of Russian letters are generally complete and 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 the letter in question. 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, understandable 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.