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Abbreviations General forum/chat/mailing list abbreviations
AFAIK - as far as I know
AFK - away from keyboard
BBL - be back later
BOHICA - Bend Over Here It Comes Again (used mostly on MP.com)
BRB - be right back
EMAAB - eat my ass and balls
FWIW - for what its worth
FYI - for your information
IMO - in my opinion
IMHO - in my humble opinion
LOL - laughing out loud
LMAO - laughing my ass off
OFB - our favorite band
OT - off topic
ROFL - rolling on the floor laughing
ROFLMAO - rolling on the floor laughing my ass off
WB - welcome back
WTF - what the f---...

Band member names
CD - Charlie Dominici
DS - Derek Sherinian
JL or JLB - James LaBrie
JM - John Myung
JP - John Petrucci
JR - Jordan Rudess
KM - Kevin Moore
MP - Mike Portnoy
MM - Mike Mangini

Album titles
WDADU - When Dream and Day Unite
IAW or I&W - Images and Words
LATM - Live at the Marquee
ACOS - A Change of Seasons
FII - Falling Into Infinity
OIAL or OIALT - Once in a LIVEtime
SFAM - Scenes From a Memory
LSFNY - Live Scenes From New York
SDOIT - Six Degrees of Inner Turbulence
TOT - Train of Thought
LAB - Live at Budokan
OV or 8V - Octavarium
SC - Systematic Chaos

Other abbreviations
Usually acronyms and/or initialisms are used for song titles, band names, album titles, etc. It usually depends on the thread of conversation as to what abbreviations mean what, but chances are if you can follow the conversation you'll be able to guess the abbreviations being used.