Today, virtually any acronym or shortcut that is commonly used in text messaging can be used on IRC. Some of the older people may not get it, but that's OK – they'll learn. Generally speaking, when talking in a channel, it's best to use full sentences, and try to use proper spelling and grammar. IRC has people from all over the world, many of whom use English as a 2nd language, and they are not familiar with current slang or text-messaging shortcuts. Let's try to be as inclusive as we can.
ap – age play
brb – be right back
bbiab – be back in a bit
bbl – be back later
bbs – be back soon
gj – good job
idts – I don't think so
ikr – I know, right?!
ircOP – an IRC network Operator – an Op that has channel access to the entire network.
ltns – long time no see!
OP – a channel operator (like a moderator). Ops will have an “@” prepended to their nicknames in channel.
rp – roleplay
ttyl – talk to you later
wb – welcome back
wd – well done
wyd – what are you doing?
;p – tongue in cheek, or sticking out your tongue. It simply mean you are joking.
🙂 – smiley face – adding a smile to show your friendly intent.
0_o – generally used to show surprise, like a what the fuck moment.
😉 – winking
}:) – devilish grin
:/ – not amused