The trend was started on the Firetree US Server by 2 Australian Horde players, It's primarily used to poke fun at and trash talk classes that quite literally "mash" one button and still win in a PvP fight, or top DPS/Heal charts in PvE.
Hunters, Death Knights, Ret Paladins, Disc Priests, Rogues are perfect examples.
Can be written as a single "mash" or many i.e "mash mash" or "mash mash mash" etc.
Can also be coupled with "Nom Nom Nom" to imply "sucking the cock" of their much loved flavor of the month class.
Examples are usually shouted at the computer screen (when losing to one of these no skill classes in PvP/PvE)
or yelled out in Trade Channel to sarcastically impersonate and slander players of the targeted class/es - Can also be considered a form of trolling in WoW.
There are many ways to use "Mash Mash Mash" to trash talk players. Be creative :)
Me: "At least i don't play a mash mash Ret Pally"
Aubrey: (to himself) "Shit, i just got owned!"
Some guy sarcastically paying out on Disc Priests in Trade: "Hey, look at me, I'm a Discipline Priest, i run around and mash mash one button (Penance) and no one dies!"
-with the Nom Nom Nom
Paying out on Death Knights and their OPness: "Look at me i Nom Nom Nom my Mash Mash Mash Death Knight, blow Army of the Dead and solo 25 Ulduar!"
Random player slandering Ret Pallies in Trade: "Look at me, i play a Ret Paladin, I Mash Mash Mash one button and everything dies. Nom Nom Nom. Oh look here comes 2 Death Knights, 1 Resto Druid and a Mage, never mind I blow Wings and Mash Mash Mash some more and end up winning, I have skill! Nom Nom Nom!"
Two Warlocks are owning up in world PvP, Drakedog style! Until a Ret Paladin, Death Knight and Disc Priest come in to stop them.
Warlock 1: "WTF! I just got 2 shot by a Relentless geared Mash Mashing Ret Pally!"
Warlock 2: "That's ok, I nearly killed that Nom Nomming Death Knight until that fucking Mash Mash Disc Priest healed him to full with one button, fucking no skill at all!"
Warlock 1: "Yeah, they couldn't handle having their alts rolled by 2 gimp locks they had to bring in their Mash Mash mains in order to win, fucking gay!"
Warlock 2: "At least we don't Nom Nom the Mash Mash classes, we have skill!"