Well, here are the common acronyms. I'm too lazy right now to get into the more FFXI specific terms and jargon.

BLM - Black Mage
BLU - Blue Mage
BRD - Bard
BST - Beast Master
COR - Corsair
DRG - Dragoon
DRK - Dark Knight
MNK - Monk
NIN - Ninja
PLD - Paladin
RDM - Red Mage
RNG - Ranger
SAM - Samurai
SMN - Summoner
THF - Thief
WAR - Warrior
WHM - White Mage

EP - Easy Prey
EM - Even Match
DC - Decent Challenge
T - Tough
VT - Very Tough
INC - Incredibly Tough
NM - Notorious Monster
HNM - Hyper Notorious Monster

AF - Artifact Armor
JSE - Job specific equipment
RSE - Race specific equipment

LS - Linkshell
PT - Party
TNL - To next level
WS - Weapon skill
AFK - Away from keyboard