Thanks a lot! That's very useful. I'll translate into English your post for the benefit of the forum:
Uniquely Ukrainian formation, which arose from 'щавити (шчавити)', possibily through the form 'розщавити (рожчавити)' as a result of translocation.
Щавити (Russian щавить), Slovakian štavit’ sa, Bulgarian щавя, Macedonian штави, Serbo-Croatian штавити – Proto-Slavic [*sъčaviti]; -- derived from *sъčava (<*sъkěva, *sъkjava) «juice»; consequently, the original meaning of the word was «to obtain (from something) juice».
This is very useful. BTW, as far as I can tell Slovakian 'štavit’ sa' means 'to bet, to gamble' (similar to Polish stawiać), so it doesn't look related to 'squeeze' to me; Google translate for the Bulgarian and Serbo-Croatian words given above gives as translation 'curry, tan, bark' but I don't know what that actually means. Also, none of the Russian dictionaries I consulted has an entry for 'щавить'.
In any case, what matters is the Proto-Slavic root; according to the Polish etymological dictionary by Boryś the Proto-Slavic root *sъcati 'to produce a liquid' is at the origin of, e.g., Polish szczaw and Russian/Ukrainian щавель 'sorrel' (a juice-rich plant, presumably -- I'm not a fan) and Polish (vulg.) szczać 'to urinate' ('frequentative' form sikać), Russian сцать/ссать, Ukrainian сця́ти and similar verbs in other Slavic languages.
So Ukrainian чавити is actually cognate with Polish szczać and Russian ссать, both deriving from an original meaning 'to produce a liquid from something'. That's a turn up for the book!
