train means to gather a whole bunch of monsters (aka mobs) and have them chase you. You take your train to someone else, and get your big pile of mobs (the train) to attack your victim. So train is a noun (the big group of mobs) and a transitive verb (bringing the train onto someone)
flag means to attack another player so that your name on the screen turns purple. It means you can be attacked without force-attack, and if you're killed while flagged, the killer does not get karma (doesn't go red).
The BloAA (Big list of abbreviations and acronyms), is great, however - it's BIG and I can't remember them all yet.
Besides there are a lot of terms used which aren't part of the BloAA, and aren't really abbreviations and/or acronyms.
Like the phrase "got owned", I suspect it's a common gaming term, but what does it mean?
(I feel like a stupid character in a kids-cartoon, which is created for the pe****gical purpose of asking silly questions:
"Christopher Robin - what exactly is the north pole?", asked Winne.)