What is the meaning of this? I have seen it in several texts.
whether their conversation "will fly thick and fast about financial collapse/crash" or not.
Long time no see!
"Fly" isn't part of the expression (though it collocates quite often with 'thick and fast'). A more common collocation is come "thick and fast" [= very frequently]. "After the baby was born, visitors came thick and fast to coo over the new arrival."
In your quote it's saying that there will be many conversations about the financial situation - it will be the topic "on everyone's lips"