555555555555 is a curse word not very commonly used, but when it is used it is 99% of the time Dota 2 players. The word is most commonly used by plebs who love to trash talk other players for various reasons.
What 555555555555 means is still unknown, but most peoplebelieve it means "Idiot dog".
An Egyptian insult, pronounced like CHHHHHH (like the grunting of a pig or a very great snort through the nose). Its a pretty bad insult and u can often expect a fight afterwards, exept u put an A7A afterwards, then it indicates that you are surprised.
Its written 555 because the 5 looks like the arabic letter خ which indicates the sound CHHH