In a previous post, I speculated on the etymology of the Russian word пост (“post”), which means “fasting”, as in Lent. The solution was provided, over on Professor Robinson’s blog, by commenter Aule Valar, who found this link. And the cognate is so obvious that it should have jumped out at me and bit me in the nose, in the very first place.
In fact, the Russian (and Slavic in general) *post is one and the same word as English “fast“, just with a different initial consonant (don’t worry about the vowel, that’s handled separately), typical of Germanic consonant shifts. We’re talkin’ Grimm’s Law, baby. And for those who think Grimm’s Law has something to do with Little Red Riding Hood… well, you’re not actually that far off the mark! In any case, just stipulate that an original Indo-European /p/ shifts, becomes toothier (like Red’s Grandma!) and becomes an /f/ in the Germanic languages. Once you admit that fact, then everything becomes Über-clear.
Moving on to the reconstruction: Since the word /post/ as “fasting” exists in all the Slavic languages, then it goes without saying that this word existed during the Proto-Slavic period. Before Slavic split up into Czech, Serbian, Russian, etc. Linguists believe that the Moravian Slavs borrowed this word from the Germans of that time, with whom they were very cozy. With the caveat that, this must have occurred before the consonant shift from /p/ to /f/ in the Germanic languages. In this regard, it should be noted that the Slavic peoples borrowed oodles of words from the Germans during this phase of their common history. This was around the year 500 AD or so, and the most cited examples are the words for Caesar (German Kaiser, Russian Tsar); helmet (Russian shlem); folk (Russian polk, “Regiment”), etc.
Taking the etymology one step back, Comparative Linguists debate among themselves, if the Germanic, e.g., *fastu- had anything to do with “fasting”, per se. The actual meaning is “firm” or “solid”, from Proto-Indo-European *past- (Sanskrit pastyam, “dwelling place”). I personally think this etymology (with the semantic shift “holding fast or firm to something”, like observing religious laws, etc.), is dubious, but you never know.
Nor does this Proto-Germanic *fast have anything to do with the homonym “fast” as in “quick”. However, in Modern English the etymology has come full circle, with “fast food” being, sort of, the very opposite of the notion of abstinence. QED.