Eternity is a long time... as in forever. This word has to do with an actual forever. So, like, forever. Not like an "oh man I haven't done that in forever," because if you've done something before, it's not forever. This is forever for realsies.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
I’ve removed my armpit hair permanently, so my armpits are very smooth now.