AE - Aggressive expansion
Best CB - Declaring a war without a casus belli. Comes from the phrase "Best CB is no CB".
Border gore - Very irregular borders: full of holes and disconnected or stretching bits, sometimes called “border snakes”. Border gore doesn’t have a real mechanical penalty, but it is aesthetically unpleasing.
To bird / to squirrel - To exit the game and load a previous save, while playing on ironman, also known as “save-scumming”. Comes from the joke "Look, a bird / a squirrel!", which is supposed to distract the viewers, while the player quits the game. Alternatively, the joke is that the squirrels ate or unplugged the cable.
Cahoots - Cohorts
CB - Casus belli, or a reason to go to war
Cheevos - Achievements
Fascist mana - Political influence. Comes from the fasces in the icon.
HC - Heavy cavalry
HI - Heavy infantry
LC - Light cavalry
LI - Light infantry
ME - Military experience, also sometimes “military mana”.
PI - Political influence
Playing tall - Focusing on internal development and population growth
Playing wide - Focusing on expansion and conquest
Siege race - When you and your enemy siege different forts. Who sieges first wins the race.
Stab - Stability. "To stab up" means to increase stability.
Stackwipe - Completely destroying another army, without letting them retreat
To vacuum pops - To gather pops from uncolonized lands as a migratory tribe, depopulating them
Vassal swarm - A strategy of playing with many vassals, who will all join the overlord's wars, acting like a big swarm
WE - War exhaustion
ZoC - Zone of control exerted by forts