Why did it start?
Being the Dreadlord is Boaster's favorite hero, one of the characters acknowledgement audio files sounds like the word "AGLEED". It is debatable on whether or not the Dreadlord actually says "AGLEED" or just agreed. Boaster coined the phrase, and it later caught on as popular mainstream word.
In Boaster's original post introducing the word "AGLEED" to everyone he said:
"From now on, instead of saying AGREED say AGLEED like the Dreadlord!"Very few players actually did at first, but as previously stated it gradually caught on as a popular mainstream neological term.