Yes, taunt. Exactly taunt and nothing else.
For a Company that refused to even show League borders of opposing team in loading screen, you sure weren't against giving players a way to taunt their opponents. And that is how people use ctrl+6 mastery thing most of the time. They get a kill, ctrl+6. They escape from a gank, ctrl+6. And if they want to be especially annoying, after getting a kill they just stand on top of your champions body and spam it. It's literally LoLs version of teabagging.
It's offensive and annoying. And worst of all, it's not simply players being %%%%s, it's Riot giving players a tool to be %%%%s and not giving us any ways to counter it. There is no "turn off mastery emote" option in settings. Nothing.
So please, add that option to the game. Ctrl+6 spam is toxic and players should have a way to turn it off.