Since Tibia started, players have created ways to say some words easier and faster. This is considered anti-roleplaying, but most people use these abbreviations in everyday conversations.
There are some roleplayers that refuse to use these words and even refuse to use chat channels. Instead they will use the default channel (if they are standing near the other person), or write letters to converse with a player that is further away.
ISO 639 / 3166 codes: real life language / country / ethnicity
ISO 3166 and ISO 639 document standards for shorthand country / language names. These are used for many applications (on the Internet especially), and as such, Tibians have largely adopted the same standards. Note that it can be used both as an adjective (a pl guild - a Polish guild) and a noun (a br - a Brazilian); further, it is often used by natives to inquire about other players' nationality. A Brazilian is likely to ask thus, "br?", asking whether the correspondent is from Brazil or speaks the language spoken there (i.e. Portuguese). Note too that it is unlikely to hear Americans, Canadians, Australians, or many Europeans asking. The reason lies in the education systems around the world, i.e. there is either a lack of English education in some areas or there is not enough foreign (non-English) language taught.
##/## - Two Numbers that means your Skills.(Melee/Shielding for Knights and Distance/Shielding for Paladins.
Example: 45/50 means 45 for attack skill (depending on the type of weapon you use) and 50 for shielding skill.
#k - A number Multiplied for 1000, normally used with Money, but also used to refer to experience points (usually how many to go for next level) or weight of a large lootbag in ounces.
Example: "I have 2k gold" means the person has 2000 gold.
Aff - Either "away from friends" (a thread, I will get you when you are alone) or an expression of annoyance, mainly used by Brazilian players.
Freeze - Happens when the Tibia Client freezes and you later can see the "history" of what that happend when you were in "freeze mode". Caused by problems with your connection. See "lag".