Answering directly to your question
this expression means basically everything, including all the sordid details you didn't want to know (TMI). Is that accurate? I would say "no", it means the truth, but with one important exception, that it is the one you would like to hide, conceal from others. Not the one you don't want to know.
From my dictionary
Правда, истина, тщательно скрываемые обстоятельства, подробности чего-либо.
The truth, verity, thoroughly concealed circumstances, detail of something
So you can translate it like
All (this stands for вся) truth, verity, thoroughly concealed circumstances, detail of something.
This article explains its etymology
Происходит от ноготь. В старину выпытывали правду путём выдирания ногтей
или засовывания под них маленьких опилок.
It derives from "a nail", because long time ago the truth was extorted via nails extirpation or shoving sawdust under the nails.
I would like to add one important note
I have never heard this word in any serious situation. What do I mean with 'serious'? Consider a situation: counterintelligence officers were interrogating some spy in order to gain some data, sort of truth from him. Even if they were torturing him, it would be very strange for me, as a native speaker, to hear something like
офицеры контрразведки допрашивали одного шпиона, чтоб узнать его
This word is usually present in colloquial language, in situations like: my representative is a substance abuser, driver under the influence, adulterer and whatever. And someone is telling me, that he is a very good man, so I can say:
да я же знаю всю его подноготную
The more proper definition of the meaning would be:
The truth, verity, thoroughly concealed circumstances, detail of
someone's life, behavior, morals.