What is Everybody?
1.
All people whose existence the speaker deigns to acknowledge. Generally limited to those with similar interests and vocabulary.
Everybody knows that Super Metroid is the greatest game ever made. (everybody = video gamers who have played Super Metroid, whose favorite genre is the adventure-platformer)
Everybody lost money in the dotcom bust. (everybody = investors in the dotcom bubble)
See
2.
Something that they sing on homestarrunner in the theme song.
Everybody! Everybody! Eveybody! La de do da do.
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