Boosted vs feeder vs bad player vs bought account. Know your terminology
Please don't insult your team. It tilts your teammates, it tilts YOURSELF, and you eventually get in trouble for it. If you're into this game so much that you get mad enough to insult people, you probably don't really want your account banned. ESPECIALLY right before the end of season rewards. Even smurfs. That's several hours of your life lost if you get your smurf banned. Anyway..
Boosted = paying a better player to win games on YOUR account. A favor from a friend would count too. If you suspect boosting, check their lolking graphs. You can usually tell where they bought the boost, if they actually did. A huge, inconsistent spike to Gold (or whatever rank), then a dip back to previous levels implies a boost (but doesn't confirm it).
Feeder = someone with a negative KDA. Not to be confused with an Intentional Feeder (intenting), who is purposely giving kills to the enemy team. Feeding can be unintentional. Everyone UNINTENTIONALLY feeds at some point. We all have our bad games, for whatever reason.
Bad player = someone who makes the same mistakes and refuses to learn from them, IMO. Or someone inexperienced. This definition is up for debate. But it's NOT someone with lower MMR than you. Lower ranked players can still be good, in their skill range. Good players have bad games, bad players have good games. Low MMR players can be knowledgeable. (Back in Season 4 or 5, one pro coach/analyst, I think MonteCristo, was Gold rank, but understood the game at a Challenger level).
Bought account = paid for someone else's account. Not always boosted account. You can buy a boosted account, but then someone else was technically boosted.
BONUS term: toxic = someone being rude or insulting. Using any of the above terms in-game, when a player is doing poorly, is typically toxic. Calling a player toxic, ISN'T toxic. Though it's impolite (I'm guilty of this though). Telling a player they're being reported isn't inherently toxic, but it's close. Intentionally feeding, taking someone else's role, or purposely throwing off a team comp with an intentionally bad pick, are all examples of toxicity. EDIT: Purposely throwing off a team comp with an intentionally bad pick is ONLY toxic if done with MALICIOUS INTENT. As in, "you took the role I called so I'm gonna ADC Galio F you". Extreme example, but that's probably toxic. However "I wanted to mid but I only play Galio, I'll take him bot and do my best" isn't toxic. Credit to Desolas Arterius for clarifying this. He made me think it over, and I was wrong in how I said it. A non toxic example, as Desolas Arterius explained, is a one trick pony taking an unorthodox pick. (This reminds me, I have a Diamond 4 friend who would play Wukong in all roles, and carry his teammates).
I've had my fair share of toxic players lately. And, most of them misuse the terminology.
tl;dr: don't be toxic. But get your terms right.