Translation:The chair that we bought is too tall.
It is unspecific. Can mean almost anything. The first thing I imagine is a chair which has the seat to high, but it can be also too tall to fit somewhere.
BTW, I think I have never used this "již" in my life. Not spoken, not written. Just to weird, old, bookish, formal, I don§t know what is the right word, but I would have to thing long to use the correct declination. Probably I would have to refer to a Czech grammar handbook.
I think he/she was referring to your (VladaFu's) comment from 2 years ago: "I think I have never used this "již" in my life".
QlIKbokf, the problem is that Czech are forgetting how to use "jenž". We all know how to use "který", but "jenž" has become very formal and not used much anymore. And since "jenž" has some unusual declination (even for Czechs), people are forgetting the correct forms. It's quite a common thing to see someone less educated, who wants to sound clever, to use "jenž" wrong - often just sticking to the form "jenž" even though the gender/number/case require a different form.
And this feminine accusative form is particularly treacherous, because it looks exactly like another "již" - the formal variant of "už" (already). So Czechs tend to process it like this: "That chair, we already bought..."... wait a minute, that makes no sense... oh, you mean "kterou"?
Many forms are easy and even uneducated Czechs sometimes get them right, e.g. masculine/neuter genitive: "bez něhož" (without which) or masculine/neuter locative: "o němž" (about which). But consider the simple masculine nominative plural - it's "již" again. E.g. "Muži, již stáli vedle mě, beze slova odešli." (Men that stood next to me have left without a word.) - Few Czechs know that the correct form is "již", and even they will avoid it and use "kteří" instead because the "již" form is used so little that it's confusing.
If it was at least clear what is proper. Well the, grammar handbooks are clear, but you do not get those forms from reading older literature. They are actually quite artificial and only started to spread around 1840 or 1850, which is quite late. For example "Matka, jenž ..." used to be completely correct before. That means that reading, which normally saves your language sense, will only help if you restrict yourself to a narrow band between 1850 and the time writers mostly switched to "který". If you read Mácha, Komenský, Hus, you will learn the "wrong" forms.