How would you interpret this saying?

It sounds like whoever is down to their last bit of luck.

Yep.. I think it means that oo.

my initial thought is that it means that if there was anything less in the glass, it would be empty. so it is very nearly at the end—what @Val123 said.

“All but empty” means almost empty.

Yeah, it might as well be empty.

It means it’s time to order another beer. Is what it means.

Thanks everyone, that’s what I thought. Just wanted to double check!

I think it means no matter if the glass actually was empty or full, all the hope and desire was still there.

It means almost empty – “all but” means almost.

Think of a coffee cup that is empty, but you can still smell the coffee in it. It’s that little.

It depends on the context, especially the following lines. It could mean what has been written above, but then it could also be optimistic and the beginning of something new:

My glass was all but empty
I knew that I had to
What with little strength and will left in me
Rise once again from the ashes
And go forth into the welcoming world

Say

@Zen Story of my life!

Almost empty but not quite. It suggests to me that there is still a little something left and handled with care, it may be possible to get by on it and even grow it. It’s not over until the fat lady sings.