In the movie Patton, the World War II general of the title at one point remarks "Rommel, you magnificent bastard! I read your book!" This is the only solid attribute of the trope that exists now, or so far as anyone can tell, ever existed in the first place. From before my time, Magnificent Bastard had long since morphed into a nebulous term that appears to mean whatever it is that the individual wants to believe it means.
That's the only sensible way to describe it. Even in the trope rename forums, I can't think of any entry that everybody agreed was the trope except for Rommel himself. These discussions had a tendency to turn bizarrely philosophical. Perhaps the strangest I ever read was that the bankers on Wall Street who caused the financial crisis were Magnificent Bastards. I'm sure there were probably others just as, if not more ridiculous than that, but it was this comparison in particular that caused me to write off any attempts to fix the trope as being hopeless.
The reasoning behind that specific example is simple- a Magnificent Bastard is a bastard who nonetheless acts with chutzpah in a grandiose manner. Ergo, bankers who destroyed the world economy for their own benefit and received no meaningful consequences as a result must be Magnificent Bastards. Which sounds sensible enough until you try to dig into the meaning, at which point the question becomes- exactly who isn't a Magnificent Bastard?
I invite you to try this out for yourself. Come up with a character, any character at all, who performs some sort of morally ambivalent act, and whose powerful confidence you admire, regardless of the results said character achieved. Can you convince yourself that this individual is not a Magnificent Bastard with the information I've given you, and using structural logic? If you can actually manage this feat, good job- you've bested just about every troper who attempted to tackle the topic of "what exactly does Magnificent Bastard mean?", including myself.
Magnificent Bastard is the ugly consequence of one kind of wiki behavior we were never able to resolve- what do you do with pages whose meanings are vague, and yet are profoundly popular because they sound so cool that everyone thinks their favorite character, concept or narrative action must be an example? Unfortunately, there's not much that really can be done- perhaps the page could have been saved with earlier intervention, perhaps not. But after a certain level of example density, further attempts to solve the problem simply became a waste of time. I attempted a complete remodel of the page at one point- no meaningful trace of this remains because however much anyone tried to tighten the description people kept interpreting it to mean "cruel character that I like".
There have been two significant consequences of the various Magnificent Bastard proposals. The first was the establishment of cyclical proposals in the trope rename forum quite similar to those we had had in YKTTW. Rather than being a final word, proposals in the trope renames forum were often brought up again, sometimes in ignorance of previous proposals. Magnificent Bastard, like many other topics, was often brought up by new forumites unaware of previous failed attempts to solve the problem.
The second more significant consequence is that these early complaints paved the way for the Trope Renames forum to later become Trope Repair Shop. We couldn't really rename Magnificent Bastard, since the only part of the trope anyone seemed to understand was the name. Even though in that specific case we failed, such overhauls had psychologically fallen under the purview of Trope Renames because, as with the YKTTW stage, it didn't seem like there were any other places to ask those questions. As was the case then, this wasn't entirely true. Nonetheless, nobody knew this at the time.