Arr, Spongebob, what's the meaning of all these nicknames?

But really, I want to know what all these things like McGar, Skarmbliss, and Tyraniboah are.

These all refer to various competitive batttling movesets. Skarmbliss is when you pair together a skarmory and a blissey and is pretty unstoppable. Tyraniboah is
Tyranitar @;leftovers
EVs: 252 HP, 48 speed, 210
Rash nature (+SA, -SD)
- Substitute
- Focus Punch
- Crunch
- Thunderbolt

mcgar is a genger specialized in countering the afor-mention skarmbliss.

They are the terms of different Pokemon with special set. Go smogon.com/dp/pokemon/XXX where XXX is the name of pokemon (Salamence if you want to find SpecsMence for example). I will list the most common sets. Just copy that little link from up, and add the name of your pokemon for proper description.

MixApe: Infernape with Mixed moveset. Nasty Plot variant counts to this too.
SpecsMence and MixMence: Salamence with Choice Specs and Special movepool. MixMence is Salamence with Mixed moveset.
ChainChomp: Garchomp with Mixed moveset. See also HaxChomp.
CursePert/Lax: Swampert or Snorlax with moveset based around Curse.
CeleTran: Combination of Celebi and Hetran who work as a wall combo.
FlinchKiss: Togekiss abusing Seren Grace with Thunder Wave / body Slam and Air Slash.
Suicide Lead: Lead pokemon that uses usually Stealth Rock and Explosion, commonly with Focus Sash.
Revenge killer: Sent out after your pokemon after another is dead. DO NOT MISUNDERSTOOD WITH COUNTER! For example, Weavile is Revenge killer to Gengar, since It outspeeds it, and OHKOs it with Pursuit even if it stays in.
Counter: Can come in and pose immidiate threat to surtant pokemon.
There are whol list of more, but these are one of the more common.