What does stacking Nasus mean?
Okay so I'm kinda interested in playing Nasus but I wanna know the basics first ... So what does stacking mean in playing Nasus, is it increasing his health bar, or some kind of increase in damage or AP? what is it? 
Okay so I'm kinda interested in playing Nasus but I wanna know the basics first ... So what does stacking mean in playing Nasus, is it increasing his health bar, or some kind of increase in damage or AP? what is it? 
's Q ability, Siphoning Strike, deals a base amount of damage PLUS a bonus amount of damage per use. You get more bonus damage on his Q every time he kills something (3 damage per unit he kills, but double that for champions, large minions [like Tibbers and Cannon Minions], and large monsters [like the red buff]. So when someone says that Nasus is stacking, or asks how many stacks Nasus has, they are referring to the bonus damage on his Q ability.
Everytime you kill a minion with your Q, the damage on it goes up by 3. If you use your Q to kill a large minion or champion, the damage goes up by 6 instead. Stacking means farming up your Q, also the damage increase is only for your Q, however because of that, people at times build full tank Nasus and use his AoE damage on top of his stacked Q to fight.
His Q.
Every time you kill something with his Q, it permanently gains 3 stacks. For every stack, his Q does 1 extra damage.
It doesn't sound like much, but if you're really good at (proxy) farming, you can easily rack up 300 CS by 30 minutes. You should be more than able to get at *least *half of that with your Q, so that's basically an extra 500+ damage per Q 30 minutes into the game.
Killing minions, monsters, and champions with his Q ability gives him Q stacks, which increase the damage of his Q. His Q can become infinitely more powerful.
It means you are screwed bro
So, the others have explained the ability already, but just as a reference try to have ~100+ at 10 minutes, ~300 at 20 minutes, and try to stay on it through the whole game. Mind you, it's a fine balance between staying in lane to farm and moving around the map doing things. That'll come with practice though.
What does stacking Nasus mean?
i could post you an image but i would be banned... this is a christian board i believe