Can be used in a varitey of contexts:
1. As a term of disappointment similar to the word lame.
2. As an insult, similar to the word "idiot" or "berk".
3. As a verb, that means you're going to do something, what that something is, is left for you to elaborate on.
1. "Hey dude I lost my phone in a canal last night when I was pissed"
2. Dude, I can't believe you puked up on my bed you fricking flerge"
3. "Im gonna flerge you"
"I flerged your mum last night and it was well lolcats"