The term "euro" is used to describe an extremely defensive style of play. It is often jokingly cited as having originated from instances where Monks for European GvG guilds would shout at their frontliners to lineback the rest of the match once any party member's health bar dipped below 90% because they had run out of energy.
Rebel Rising is a historically "euro" guild, running builds with up to 6 primarily defensive characters with a single skill devoted to spiking out opponents. "Euro" playstyles became extremely popular following much power creep and several Rebel Rising mAT victories.
It can also be used as a derogatory term to describe anyone currently residing in a European country.