On August 17,2013 Greg first used the term "smurfs" in his "ONISION PROVEN INNOCENT!!! #ApologizeToOnision" video. In the video Greg declares his innocence against the commenters, bloggers,and vloggers that claimed he was responsible for Reptar's death. In his rant he calls his critics "smurfs" and explains this is because they dont deserve names. Later in the video, he refers to anti-onision blogs as smurf blogs. In the description, he tells viewers that believed he killed his tortise "So smurf you, smurf you, and yeah smurf you too." ($ource) This is most likley the first time Greg used the term smurf.
Following the video Greg used the term on social media. (Source)