Disguise

There's a tradition in drama and comedy of a lead character disguising himself (or possibly herself) and hearing truths about themselves that they would never normally discover (think Henry V walking amongst his troops). This may be the cause of the old expression that eavesdroppers never heard anything good about themselves.

That's fairly harsh. Are they suggesting that the kind of person who would eavesdrop is clearly a cunt, and therefore all their friends will be saying: "Geoff, he's such a fuckface. And he's an eavesdropper, the weaselly shit, I hacked into his emails and read him talking about it.What a misshapen cock."? Or is it just supposed to be bad luck, so that if you eavesdrop it will be the one moment when someone is saying "... and he fucks badgers..." and then you run away screaming and miss the bit where they add "...but no one minds cos he's so nice the rest of the time and who doesn't occasionally want to violate a medium sized omnivorous mammal anyway, I say fair enough."?

It's a fucking minefield.