second guessing ...

what does it actually mean ?

is it, as i suspect, nothing - a meaningless bit of jargon like
“leveraging synergies” etc ?

It means to critique a decision after the results of the decision are known. The person who made the decision had the first guess, and now someone else is making a second guess but with the benefit of knowing everything that happened.

Also known as Monday Morning Quarterbacking.

I thought it was more along the line of critiquing a decision before the results are knowen. Or after the results are knowen but the consequences have yet to show themselves. For example, going to buy a new car, going to the lot, talking to a salesperson, but then wondering if you really do need a new car right now. OR Actually buying a new car, and then a day or so later wondering if it really was the right car for you.

Of course, dictionaries can come in handy sometimes.

So CookingWithGas is correct, and Joey P is also correct with the first part of his answer.

The second part of Joey P’s answer refers to something else - “second thoughts”.

Or in Joey P’s example, buyer’s remorse.