So, these are, for sure, technical terms or slang terms (depending on your perspective).
A little history lesson... most of the games these days which have people running around shooting other things (either with plasma weapons, regular weapons, or bows and arrows) use a mechanism to track a character's health, and almost always the term for this inside the game engine is: Hit Points.
Now, that term should be very familiar to any who play Paper and Pencil Roleplaying Games and it is, indeed, the exact same mechanic. It's just an abstract number that represents one's remaining "life".
So, with that history lesson in place:
What does very dead and extremely dead mean?
It means that the target suffered so much damage that they exceeded their damage potential by a significantly wide margin (i.e. if they had 10 health, they're now at -20... they're very dead.)
By way of example (a very old example), in the game Quake, a character suffering from massive damage would be "Gibbed"... I shall not explain what means, you may look that up. That term has been replaced with "Very Dead" and "Extremely Dead" as most games do not have specific mechanics for this scenario.
https://quakewiki.org/wiki/Gib
"... bringing an entity's health below a certain threshold, for example -40 for a player, or..."