An epiphany is an event where in a flash a person realizes the truth of a situation or when something that made no sense or that was thought to have no significance is suddenly understood all at once.
The Epiphany described in the New Testament was the visitation of the wise men to honor Jesus when he was a small baby. That was a moment when the outside word first began to understand Jesus significance and Jesus and Mary were confirmed in their own beliefs.
Another epiphany that Mary and Joseph experienced that caused them to believe that Jesus was quite unique and was called to do something special occurred when they visited the Temple as a small boy, and he participated in the disputations that learned men were having there, and stayed in Jerusalem at the temple even when his party was returning home.
Novelist James Joyce especially generalized from specific events reported in the Bible to other life-significant events that were interesting to a storyteller or novelist. See this article on epiphanies in Joyce and Wordsworth's work.
If you hear someone using this phrase in conversation they likely have literary or critical training. Qualifying it as a "first-person epiphany" is evidence of this particular writer's cleverness. This whole phrase is not common usage.
If you used it, you would likely have to explain that you mean a flash realization that a person has about his own situation or predicament.
Might be OK to use epiphany in the vicinity of a University or among friends who were writers, English students or divinity school atendees. Could get you heckled or worse at a sports bar.
I've usually heard this as "eat paste", but "eat glue" is probably the same thing, especially judging from the informative graphic.
This refers to doing something passively stupid, and hearkens back to the kid in grammar school who merely sat at his desk eating paste while things happened around him (or her).
The graphic is depicting the Internet Explorer web browser as the dumb kid who has, by inaction, removed himself from the battle over web standards, which is hotly being fought between Firefox and Chrome. With the release of IE9, I'm not sure how applicable this is anymore. In any case, IE is represented as being content to just be what it is (with still a majority market share) while the other browsers fight it out for what's left.
It's not very flattering to Microsoft, certainly, but it also suggests that it might be better to be the dumb kid who wins than the smart kids who fight each other.
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A more common idiom is being knee deep or up to your knees in something, meaning that something reaches one's knees. When you are up to your knees in water, you have gone in to a point that you are definitely wet, and more importantly, there is so much water around, that walking gets more difficult (you move from walking to wading).
It is applied to other things as well, meaning that you are surrounded by a large amount of it, literally or figuratively (knee deep in sh!t means you are in a bunch of trouble).
In the same way, the person who said We'll be up to our butts in corpses means that once they are done with what they are going to do, corpses will be piled up around them up to the level of their behinds. Whether that will be literally the case is not relevant: the message is that there will be a fair amount of dead people, once they are done.
Note that the sentence as you quoted it is ungrammatical. It may be a typo on your side or an oversight of the editor, but it should either be we'll be up to our butts in corpses, or I'll be up to my but in corpses. It escapes me why one person would refer to multiple butts in this way.