How can the expression "高就" be understood grammatically?
If you are looking for an exact English translation, the answer is that there isn't one.

It is a tradition in Chinese culture. When you are talking about/to other people, you should butter him/her up by exaggerating his/her achievements, wealth, position and etc. And do the opposite thing when talking about yourself. It's all about being polite (when talking about others) and modest (when talking about yourself).

An example conversation may be like this:


A: 您在哪里高就呢？ (So I heard that you've got an excellent job now. Where is it again? )
B: 哪里哪里，不过就是在Google混口饭吃而已。 (Well, I work at Google, just barely getting by.) 


A called B's job an "excellent job(高就)", not necessarily for the literal meaning, but just being polite. And B said that he is "just getting by"(混饭吃/混口饭吃), which is not necessarily true (we all know Google pays a lot) but just being modest.
The translation above isn't word-by-word. But you get the gist. You can also see that you can't talk to a native English speaker like that.

If you say: 


我在Google高就
听说你在Google混饭吃


While these sentences are grammarly correct, they are simply wrong.

So don't bother with the translation or the grammar. Just understand the cultural difference behind this. 

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If you are really into grammar, it's something like this:

高(adv. better)就(v. work)

您(在哪里)高就 (Where are you working?)

To be culturally correct, 高 should be lost in translation.