Relation between Russian "пока" and Czech "zatím"

Question

I have noticed that the Russian word пока means the same as zatím in Czech in both meanings. The first is as a conjunction and the second use means goodbye. I am aware that in Czech the equivalent expression to пока as a conjunction is "zatím co" but I still think this is a very interesting coincidence.

Example of the first use:

• он смотрел телевизор пока она спала
• on se díval na televizi, zatím co ona spala

The second use is a word meaning goodbye.

I also know that Russian has за тем which has the same meaning as Czech zatím but cannot be used as a greeting. (EDIT: This is incorrect, see @Yellow Sky's answer)

So, the question is, how did it come so, that these seemingly unrelated words have the same two uses. I tried looking up both of them in Wiktionary with no success but this is where my etymology skills end.

Answer 1

First, about some things which are not very true in the question.
The Russian за тем means “behind/after that ...” like in за тем домом “behind that house”. There is a Russian adverb затем “then, next” which also has an archaic meaning “because, in order to” as an answer to зачем? “why? for what reason/purpose?” and it is this затем that has some semantic connection with the Czech zatím, but the meaning and the usage are quite different.

Zatímco / zatím co is a conjunction, usually written together with co, while zatím is an adverb. Пока can function as both a conjunction and an adverb.

The reason why both пока and zatím can mean “goodbye” is quite simple: пока means “so long as, while” and zatím co also means “so long as, while”, so it is not very strange that they both mean “so long = goodbye”.

A more interesting question is how and why “so long” came to mean “goodbye”. Online Etymology Dictionary explains it by German influence, and Czech had been heavily influenced by German. Still, the meaning of most parting expressions is a wish of well-being or a hope to meet again, so “be well so long as we are parted” shortened to “so long” is a connection with “goodbye”.