Forever
Eternity
noun, な adjective, の adjective
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.
えいきゅう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
脇毛を永久脱毛したから、今脇はツルツルだよ。
I’ve removed my armpit hair permanently, so my armpits are very smooth now.