A benchwarmer is a player on a sports team who hardly ever gets to play. They sit on the substitutes' bench game after game, waiting to be called on — and the joke in the word is that the only thing they are doing is keeping the bench warm with their body. It is informal and a little unkind. Calling a teammate a benchwarmer is teasing at best and an insult at worst; you would not use it in a serious sports report, which would say substitute or reserve instead. The word came out of American sport — baseball and American football especially — and travelled from there, so it turns up in basketball and football talk everywhere now. You will see it written benchwarmer, bench-warmer, or bench warmer; all three are fine. It also gets used outside sport, for anyone kept on the sidelines at work — someone hired but never given real work is a benchwarmer too. Note that this is a person, not a thing. It never means a cushion.