Abbreviations commonly used by stampers (original list courtesy of the fabulous Jean Fitch, shortened a bit by me)

ABC = Anything But a Card
ATC = Artist Trading Cards - It can be anything the only thing is that it MUST be the size of 2.5" x 3.5" or 89 x 64 mm.
ATG = Adhesive Transfer "Gun" made by 3M
Avatar = the photo/icon associated with usernames
BG = Back Ground as in BackGround Stamps
BIA = Bind it All – binding machine
BP = Blender Pen
BRAK = Birthday RAK
Brown – UPS (go figure, a longer word for a shorter acronym…)
BS – Big Shot (except of course for when it means the other Bovine thing – LOL!) – Sizzix die cut machine sold by Stampin’ Up!
BST = Buy/Sell/Trade
CAS = Clean & Simple - card style - minimalistic
CASE = Copy and Share Everything
CATTY = Catalog
CB - Cbug = Cuttlebug – a die cutting machine
CE = Crystal Effects and/or Current Events Forum
CL or CM = Clear Mount stamps (SU brand )
CS = Cardstock
DD = Darling Daughter or Definitely Decorative (SU stamp design and alternative term for DĂ©cor Elements Catalog)
DS = Demonstrator Support for SU demo’s
DSP =  Designer Series Paper (SU term)
DTP = Direct to Paper – such as applying an ink pad/spot direct to paper without a stamp involved
EB = Embossing Buddy
ENVY or ENVIE = Envelope
EP = Embossing Powder
IB&C or IBC = Stampin Up! Idea Book and Catalog
LO = Lay Out – generally used by scrapbookers
LOL = Laugh Out Loud
LSS = Local Scrapbook/Stamping Store
MCP = Magic Colored Pencil – technique with pencils, blending tool (stump) and Odorless mineral spirits
MDS = My Digital Studio - SU! digital design software
MHO = Mostly Hands On
MIM = More is More
NSR = Not Stamping Related
NUM = New Unmounted
NVR = No Vendor Restrictions (often used in swaps)
OMS = Odorless Mineral Spirits
OSW = One Sheet Wonder
PP = Printed Paper
PSE = PhotoShop Elements
PTI = Paper Trey Inks (stamp company)
QOTD = Question of the Day
RAK = Random Acts of Kindness – often a card sent to someone not expecting it
SA = Stampin’ Addicts – All SU Online stamping community website
SAB = Sale a Bration – Stampin’ Up! promotion
SABLE = Stash Accrual Beyond Life Expectancy
SAS = Stamp a Stack
SASE = Self-Addressed Stamped Envelope
SC = Stampin’ Connection – the SU! company community site for demos
SCS = Split Coast Stampers – stamping community website
SM = Stampin’ Memories – (SU! term for “best choice for scrapbooking”)
SNAIL – Simple Neat Affordable Inline adhesive
SS = Sticky Strip, Stamp Set, Stampin’ Success (SU demo magazine) – depends on the context
SU! = Stampin’ Up! of course!
SUDSOL = Simply Unique Demonstrators Sharing On-Line – subscription based stamping community & business training site  for Demonstrators
SUO = Stampin’ Up! Only
TAC = The Angel Company
TFL = Thanks For Looking
TFS = Thanks For Sharing
THO = Totally Hands On ( a workshop where ALL the projects are make and takes)
TV = Trade Value
UM = Unmounted
UTEE = Ultra Thick Embossing Powder
VM = Versa Mark
VV = Very Vanilla – SU color
WM = Wood Mount (as in stamps)
WRAK = Welcome RAK
WW = Whisper White – SU color
WWC = SU's Watercolor Wonder Crayons
WS = Workshop
YAGE = Yet Another Grand Exit
YILM = Yes, I Love Myself