Can somebody help me define Toxic Gameplay?

It's whatever Riot wants it to be for a given instance on a given day.

"{You} suck." The three person group responds with "Toxic! Reported!" Riot's response "Your recent gameplay is...."

As stated above, that's pretty close too. It's mostly about the kids who can't /ignore, that's toxicity.

toxic gameplay (rather than general toxicity which most often referes to chat behovior) means things like picking highly off-meta or just plain bad champs (teemo jungle etc), poor item builds, taking revive (well can't do that anymore sorry hinduism), etc