Update: If you find an acronym that is not listed on here, please feel free to contribute it to the comments and I will add it to the list. This is intended to be a "living document" for the community, so please feel free to contribute!
Abby - Abyslonia, Terror of Everblight.
Argi - Argus, pl. Never seen. Opposite of auto-include.
Barney - Bloody Barnabus. He loves you, you love him.
BnB - Bethayne and Belphagor.
Connie - Constance Blaze.
Denny - Deneghra. Can be p or e.
Doomy - Hoarluk Doomshaper. Can be p or e.
Fatty - Dominar Rasheth. Could also be a Bloat Thrall, in the context of Cryx.
Dr. Frankenpig - Dr. Arkadius
Gaspy - Lich Lord Asphixious. Can be p or e.
Gunny - Captain Gunnbjorn.
Harby - Harbinger of Menoth.
Hexy - Lord Hexeris. Variation: Sexy Hexy and Sexeris.
Chicken legs - Vice Scrutator Vindictus. Specifically refers to the thin legs on the model.
Mechano-ham - Kommander Harkevich, the Iron Wolf. Specifically refers to the weapon carried by Harkevich, and it's similar shape to a chunk of ham.
p, e/1, 2, 3 - Prime/primal, epic versions of a warcaster/warlock (i.e. pNemo vs. eNemo, or Nemo1 vs. Nemo2). The 1, 2, 3 notation has become more popular with the future addition of "legendary" warcasters/warlocks.
Scavvy - Lord Exhumator Scaverous.
Thaggy - Thagarosh. Can be p or e.
Threemo - Artificer General Nemo. This is a combination of the '3' notation above and Nemo's namesake.
TOM - Testament of Menoth.
Tree - Cassius the Oathkeeper and Wurmwood, Tree of Fate. Refers specifically to Wurmwood. This may be adopted in the future to refer to Gallows Groves.
Angel - Angelius. Typically pluralized.
BBack/Bbtitan - Bronzeback Titan.
[Bone] Chicken - Cryx light warjack type, typically refers to any of the following: Deathripper, Defiler, Nightwretch, Ripjaw, and occasionally the Scavenger.
Carnie - Carnivean.
Coach - Titan Gladiator. Specifically refers to usage with Molik Karn, to increased his threat range.
Cricket - Arcantrik Force Generator.Disco Ball - Storm Strider.
EB/EBDT - Earthborn Dire Troll.
Glad - Skorne Titan Gladiator.
Gunbunny - Grundback Gunner or Blaster.
Molik Karn - Typically mistaken for a character warbeast, he is actually a Hordes faction, typically seen with a red and gold color scheme. Example: "What are you playing today, Steve?" "Molik Karn."
Tibbers, Tabbaz - Skorne Character Warbeast Tiberion. Also refers to the name of the muderous bear which belongs to Annie, a League of Legends character.
Tessie - Cygnar Storm Strider. Likely short for "Tesla", which is alluded by the Storm Strider's affinity for throwing lightning.
Twin Towers - Two Legion Thrones of Everblight. Particularly refers to a popular list with Rhyas, featuring both 'towering' battle engines.
Webcam - Celestial Fulcrum.
AG - Ancestral Guardian.
ATGM - Arcane Tempest Gun Mages.
B13 - The Black 13th Gun Mage Strike Team.
Banes - Bane Thralls or Bane Knights, depending on context.
BLT - Bane Lord Tartarus. See also: Tartarsauce
BFS - Black Frost Shard.
Blood Pot - Spawning Vessel.
[The] Book - The Covenant of Menoth.
Darragh Wrathe - (pronunciation varies greatly)
Digmies - Pyg Burrowers.
Doggie/Puppy - Khador War Dog.
Fatty - Bloat Thrall. Also: Dominar Rasheth, in the context of Skorne.
Harm - Typically refers specifically to the spell Kiss of Lyliss, which causes the target to take more damage from attacks, seen on Lady Aiyanna and Blackfrost Shard.
IFP - Khador Iron Fang Pikemen.
ILO - Cryx Iron Lich Overseer.
Junior (Jr.) - Cygnar Journeyman Warcaster.
KEE - Knights Exemplar Errants.
Kermit/Kremit - Croak Hunter. It's not easy being green.
Kovnik Joe - Kovnik Josef Grigorovich. See also: WGDS.
KSB - Trollbloods Kriel Stone Bearer
LoF/LotF - Circle Orboros Lord of the Feast
McThralls - Cryx Mechanithralls.
Moneybags - Lord Rockbottom.
Moses - Trollkin Runebearer. Refers specifically to his resemblance to the Biblical character from the Old Testament, known for his affinity for stone tablets and yelling (?).
MOW - Khador Man-o-War troopers. Can either been Demolition Corps, Shocktroopers, or Bombardiers.
PGBH/M - Pain Giver Beast Handlers. Sometimes seen as PGBM or "Pain Giver Beast Masters".
Shrimp - Druid Stoneward and Woldstalkers. Refers specifically to the Woldstalkers, which resemble shrimp. Possibly also "prawns" in Australia or New Zealand.
SoB - Sons of Bragg.
SSC - Stone Scribe Chronicler
The Stone - Krielstone Bearer and Stone Scribes.
Stormnoun - Refers to the set of Cygnar storm knight infantry, including Stormblades, Stormguard, and/or Storm Lances.
Tartarsauce - Bane Lord Tartarus. Believed to be an auto-include for Cryx players.
TFG - Protectorate of Menoth Temple Flameguard.
WGDS - Winter Guard Death Star. Typically a full unit of Winter Guard Infantry with Kovnik Josef Grigorovich nearby. Believed to be an auto-include for Khador.
Wishnailer - Sylys Wyshnylyrr. Often mocked for having the most "y"s of any model/unit.
WoO - Wolves of Orboros.
Auto-include - A model/unit that is believed to be necessary for every list.
FA - Field Allowance.
FF - Friendly faction.
MUA - Mercenary Unit Attachment.
Pillow-fisted - A weak hitter in melee. More common among warbeasts, typically a heavy with P+S 14 or less.
Pop-n-Drop - A win condition that involves "popping" a feat (typically with pKreoss), followed by killing - "dropping" - the enemy warcaster.
SR, SR2012 - Steamroller tournament rules for a given year. Example: "SR2011 rules were largely successful, but come January 2012, the SR2012 rules were released with new changes."
Skornergy - A term coined in Mk. I where a perceived synergistic effect is achieved after its desired impact has already resolved. This was most common in the Skorne faction in Hordes Mk I.
UA - Unit Attachment.
WA - Weapon Attachment.
Aggro - Aggravator (Dr. Arkadius).
AS - Arcane Shield (Various Cygnar, Jr.).
BDtH - Batten Down the Hatches (Lord Carver, Broadsides Bart).
CG - Crippling Grasp (Deneghra, Dr. Arkadius).
CoS - Curse of Shadows (Cassius, Mohsar, various Cryx).
DW - Defenders Ward/Death Ward, depending on context (Menoth, Skorne for the former, Cryx for the latter).
FE - Forced Evolution (Dr. Arkadius, eKaya, Absylonia).
FoC - Flesh of Clay (Wold Guardian)
IF - Iron Flesh (Various Khador, Bloody Barnabus)
RoTE/Roots - Roots of the Earth (eBaldur).
SG - Spiny Growth (Bull Snapper, Carnivean).
SoG - Strength of Granite (Gorten)
SS - Stone Skin (pBaldur)
[Updated 7/10/12 - added Threemo, Twin Towers, Tabbaz]
[Updated 5/3/12 - added McThralls, ILO, Scavvy, Doggie/Puppy]
[Updated 4/12/12 - added Tessie, Aggro, Banes, MUA, Glad, Tibbers]
[Updated 2/13/12 - added KSB, SSC, EB]
[Updated 2/7/12 - added BLT, Bback/Bbtitan]