What does it mean?

If a company goes under, it has failed, or gone bankrupt. I'm guessing the expression comes from shipping. If a ship goes under (the water), it's sunk, much like a business.

Iain

It can be seen in this light:

Since the idiom "go under" means:

(1) Sink
(2) Fail
(See Cambridge Advanced Learner's Dictionary)
So "to go under" may means:
(1) To sink
(2) To fail

In other dictionaries in my hand, "go under" means:
(1) Sink
(2) Fail (The firm has gone under.)
(3) die (Slang, chiefly American)
So "to go under" means:
...
...
(3) to die

Could you possibly mean "undergo"?

He had undergone great hardships, but survived.

She was nervous about undergoing surgery the next day.

In both cases "undergo" implies facing a large problems.

Thank you!!!
I'd like to thank all the people that have answered to my question. Now there's another problem. The sentence is:"Would it ..... you if I come tomorrow?" What verb can I use? Is it better if I use "fit" or "suit"?

Would it be okay with you I come round tomorrow?

Iain

Hermione, dduck's answer is correct, but it is informal. In answer to your question, I would suggest "suit". That is also correct and more formal.

Welcome to A2K!

Hermoine, Welcome to a2k. D'artagnan is right. "Suit" is the appropriate word in the example. "Fit" is incorrect.