"Pootis" is a quote from the Heavy, from Valve's free-to-play Team Fortress 2 game. The original quote by said Heavy is "Put dispenser here". However, due to his accent, it appears to sound like he is saying "Pootis spencer here". Therefore, people have grabbed the word "Pootis" from this, thus creating an internet meme.
Pootis is the dark slime that seeps down your throat as you watch Heavy swaying left to right, you want to blink but your eyes start to bleed in the painful montion of movement as you cannot look away, you start to clench your fists as you cannot move from your current position, the eyes of the Scout venture deep inside your soul as you begin to sweat uncontrollably, and as you begin to feel dizzy the video continues and continues. The only way to stop this...is to remove pootis.