CC - Crowd control (but in this context actually means stuns)
TT = Twisted Treeline
TF = Twisted Fate
WW = Warwick
GP = Gangplank
MK = Mordekaiser
Gl & hf = Good luck have fun
U2 = You too - most common response to the one above
bb = bye bye
ditto = you too aswell
AD = Attack Damage
AP = Ability Power
ArPen = Armor Penetration
MR = Magic Resistance
MPen = Magic Penetration
Reds = Marks
Yellows = Glyphs
Blues = Seals
Quints = Big fat runes
DPS = (Originally: Damage per second) Here: Classes relying on their autoattack to do damage - ie sustained damage over time through autoattack
CD = Cooldown
CDR = Cooldown Reduction
AS = Attack Speed
LS = Life steal
AoE = Area of Effect (abilities that effect a certain area, like annie's Tibbers, or akali's Twillight Shroud
DD = Direct Damage
AA = Auto Attack
JitB = Jack in the Box (Shaco skill)
RNG = Random Number Generator - alot of games are based on this concept (best example is critting, you have x% of crit, the computer then rolls, if you hit under your x% of crit, you attack will crit, if you hit over the x% of crit you will NOT crit)
FB = Usually refers to first blood early game (obviously before it is taken) and later game can mean fall back!
OP = Overpowered
UP = Underpowered
WP = Well played
GG = Good Game
BG = Bad Game
VGG = Very Good Game
GJ = Good Job
Feeder = Someone who dies alot to the other team hence "feeding" them gold
BP = Blue pill, a left over term from beta, means teleporting back to base
TP = Teleporting either to another lane or to base
OOM = Out of Mana
BRB = Be right back
AFK = Away from keyboard
Ult = Ultimate, usually used with inc to warn due to a dangerous ultimate being near.
INC = Incomming
Mid / Top / Bot = The three lanes of SR, obviously only top and bot in TT
Prem = Premade
SS = miss something often followed by the champion name
MIA = Missing in Action
OMW = On my way
ASAP = As soon as possible
Care = Means someone might approach if you are pushing to much, often followed by pings as to from where.
B = if pinged on you means fallback, in laning phase usually means a champion that has been back in base is back to relieve you.
Re = Term used after seeing champions stated missing again.
RoA = Rod of Ages
Zhonyas = Zhonya's Ring
FoN = Force of Nature
Pots = Potions, both mana / hp ones
Oracles = Oracles potion, used to spot stealthers
RS = Rylai's Scepter
GA = Guardian Angel
SotD = Sword of the Divine
SotO = Sword of the Occult
IE = Infinity Edge
BV = Banshee's Veil
Wards = Reffering to the two types of wards, vision wards (can detect stealth) and sight wards (can not detect stealth).
Baron/Dragon = The legendary monsters on SR giving a teamwide buff, experience and gold
Golem / Lizard / Wolves / Ghouls = The blue/red/green/grey single buffs
Silence = A debuff which prevents any abilities being used by a champion (summoner spells excluded)
Farm = Farming involves the amassing of gold for player. This is usually best done by killing a lot of creeps and killing them very quickly.
Build = how a champion and its summoner is built. For champions this may includes the items one buys on a champion, and the order one gets a champion's skills. For summoners, this refers to their masteries and rune selections
Carry = A character who, if played properly, wins late game fights for a team. Typically these heroes are weak early game and require a lot of farming.
Tank = Character with a lot of HP and likely armor/resistance. Whose job is to initiate battles with the enemy players/turrets as to draw damage onto himself and protect his fellow team mates
Support = Characters who job it is to support other characters: Healers, buffers, etc.