Added a few 
Abbreviations
Eating Disorder Related
AN - Anorexia
BN- Bulimia
B/P - Binge/Purge
COE - Compulsive Over Eating
COED - Compulsive Over Eating Disorder
ED - Eating disorder
EDNOS- Eating disorder not otherwise specified
Self-Harm Related
DSH - Deliberate self harm or deliberate self harming
SH - Self harm or self harming
SI - Self injury or self injuring
OD - Overdose
Diagnosis (Dx)
AD - Anxiety Disorder or Anti-Depressant
BP - Bi-Polar
BPD - Borderline Personality Disorder, also Bi Polar Disorder
CPTSD - Complex PTSD (Similar to PTSD)
DID- Dissociative identity disorder
GAD- Generalized anxiety disorder
OCD - Obsessive compulsive disorder
PTSD - Post traumatic stress disorder
SAD- Social anxiety disorder
Medical/Treatment
A&E- Accident and emergency
CAMHS - Child and Adolescent Mental Health Service
CBT - Cognitive Behavioural Therapy
CMHT - Community Mental Health Team
CPN - Community Psychiatric Nurse
DBT - Dialectical Behaviour Therapy
DHHS- Department of Heath and Human Services (US)
DSM- Diagnostic and Statistical Manual of Mental disorders
EIP/EIT - Early Intervention in Psychosis/Early Intervention Team
EMS- Emergency medical services
ER- Emergency room
GP- General practitioner
ICU- Intensive care unit
IOP- Intensive outpatient
IP- Inpatient
MHC- Mental health center
OH - Occupational Health
OT- Occupational therapy
PCP- Primary care physician
RN- Registered Nurse
Rx- Prescription
SSRI- Selective Serotonin Reuptake Inhibitor (a type of antidepressant
SW - Social Worker
Labels/Warnings
Adult - Mature/Sexual content
SA - Sexual Assault/Sexual Abuse
SU/SUI - Suicide, although tags shouldn't actually say SU/SUI
Trig - Triggering
Forum Related
FAA - First Aid Advisor
LH - Live Help
Mod- Moderator
OP - Original Poster
PR - Post Report
Forum Names
CC - Creative Corner (board)
E&MU - Events and Meet Ups board
FA or FAF - First Aid Forum(board)
F&D - Fun and Distractions (board)
GC - General Chat (board)
Intros - Introductions board
N&D - News and Debate (board)
SD or Serious - Serious Discussion (board)
SID - Self Injury Discussion (board)
R&R - Reviews and Recommendations board, generally
R&V - Ranting and Venting board
Vets - Veterans forum, and those who post there
Slang
BF - Boyfriend
BTW- By the way
FTW - For the win
GF - Girlfriend
IMO - In my opinion
IMHO - In my honest opinion
QFT - Quoted for truth
OH - Other half
TBH - To be honest